God
A god (or deity or divinity, or goddess, if female) is an immortal being of immense power, qualitatively superior to that of the vast majority of inhabitants of the world or cosmos where it exists. Indeed, gods are often said to be omnipotence and omniscient, and while true omnipotence and omniscience may be chimeras, gods are as close as it is possible to get. On some worlds, only a single god may exist, but more often where there is one god, there is a proliferation of them—though some gods may claim to be the only true god, and denounce all others as charlatans and deceivers.
Mortals often worship gods, praising them and serving them and hoping in return to be blessed in some way with their favor. (Indeed, while not all gods are worshipped by mortals, if there is any question whether a being qualifies as a god the fact that it is worshiped by mortals is a strong point in its favor.) Many gods do indeed return their worshippers' devotion with some reward, be it one as simple as a little extra good fortune, though some distant gods may care nothing for what mortals think of them. Other gods of more malevolent demesne or disposition may be propitiated in the hopes of avoiding their wrath rather than actually worshipped.
The study of gods is called theology; a specialist in this study is a theologist.
Worship
The worship of mortals by gods is so common that it's sometimes considered a necessary characteristic of a god; many people hold that gods must be worshipped by mortals (or at least propitiated), by definition. Most celemologists don't hold with this definition; there are many beings that are not worshipped by mortals to any significant extent but which are still considered gods. Still, such worship is very common. The body of worshippers pertaining to a given god is known as a church or a religion, the distinction between the two being subtle and varying in different areas, though often the latter is broader than the former, a religion comprising a number of churches that worship different gods or in slightly different ways. (The former word may also refer to a physical building where worshippers gather.)
Many mortals worship large groups of gods, or even pay some tribute to all gods of which they are aware. This practice is known as polytheism. It contrasts with henotheism, in which a mortal acknowledges the existence of other gods, but chooses only one as the focus of his own worship. (Serial henotheism—the worship of only one god at a time, but not always the same god—is called kathenotheism.) More extreme is monotheism, in which a mortal not only worships only one god, but believes—rightly or wrongly—that his god is the only one to exist in his world. Leipotheism is the practice of not worshipping any gods—this merely refers to a lack of worship or devotion, and is not the same thing as either misotheism, active opposition to gods, or atheism, a lack of belief in gods. (Misotheism, in turn, should not be confused with antitheism, opposition to particular gods rather than to gods in general.) Something of a pathological case is autotheism, the worship of oneself as a god.
Mortals may worship gods in many different ways. They may worship the gods individually or as families, or gather to worship and praise the gods at churches, temples and shrines provided for that purpose. They may petition the gods through prayer or through reciting litanies or the singing of hymns. They may demonstrate their faith through rituals that vary by religion, and by fasts and other shows of devotion. In some cases, they may make sacrifices to the gods, ranging from purely symbolic sacrifices of cheap goods to sacrifice of valuables and even of living things, not necessarily excluding humans and other ellogous beings. Organized worship may be headed by designated officials called priests; priesthood is often a full-time job subsidized by donations from other followers, or from investments by the church.
In return, most gods bless their worshippers with certain boons. They may simply send good feelings, but more often they do reward their faithful with more material benefits, turning fortune their way in matters corresponding to the gods' field. Even gods who do not otherwise recompense their flock generally respond to specific requests when their worshippers implore them—or at the very least may spare their faithful from their wrath. Some worshippers may be granted a small portion of the gods' power, imbuing them with some spellcasting ability or other powers of their own.
What the gods get out of mortal worship is more of an open question. It could simply be in many cases that the worship appeals to their ego, and nothing more than that. In many cases, however, the gods actually gain power from worship, and a god whom mortals cease to worship may dwindle and even vanish, or lose his divinity. Gods also often have the power to designate the ultimate fates of the souls of their worshippers, either absorbing them, reincarnating them as some form of life concordant with how they lived, or shunting them off to appropriate eschatological planes, among other possibilities. In some cases, the souls of all mortals of a given region or world are subject to the judgment of some group of gods when they die. Since most gods are likely to judge their worshippers more generously than the faithless, this of course gives mortals of those regions and worlds all the more motivation to worship the gods who hold their postmortal
It may be on some worlds that gods themselves worship other, still greater beings, though one might well question what would be greater than a virtually omnipotent being. Theologists and celemologists sometimes call these beings worshipped by the gods preterdeities, or overgods.
Fields and symbols
Gods often have particular areas of dominion, regarding which their powers are especially potent. It is matters related to these dominions with regards to which mortals are especially likely to petition them. Inhabitants of various worlds may refer to these specialized areas of interest by such terms as "demesnes", "domains", "fields", "orbits", "purviews", or "spheres", but most celemologists refer to them simply as fields. While many people may think of gods as being defined by their fields, not all gods have fields, some gods differentiating themselves more by their personalities or methods than by explicit demesnes. Other gods may have multiple fields, and of course even gods with the same field may approach that field in very different manners.
Some common fields include death, war, the sun, love, and nature. The possibilities for fields are limitless, however, and on worlds with large numbers of gods the fields of some of the gods may be extremely narrow and esoteric. There may be gods of particular species of organism, of common objects and materials, of specific emotions, concepts, and phenomena. Territorial gods are common, gods who reign over specific areas, be they continents, nations, rivers, or cities; on some worlds, there may be petty gods pertaining to some rocks and trees. A god need not necessarily confine himself to a single field; many gods oversee combinations of any number of fields which may or may not have any obvious connection to each other.
Many gods have various symbols and associations, often used by mortal worshippers to decorate their temples and emblazon priestly garments. These symbols may include one or more insignia, often related in some way to the god's field, but gods may have other symbols as well: associated animals, plants, colors, weapons, and more. It is also common for gods to have one or more holy days associated with them, which may or may not be the anniversaries of major events related to the god or their church. These holy days frequently are times for special rituals or commemorations among the god's faithful, and in the case of gods or particular cultural importance (or former cultural importance) they may even become recognized holidays among the general population; over time, the original religious significance of such holidays may become downplayed or even forgotten.
Aspects and avatars
It is not uncommon for gods to have multiple aspects, different facets of the god devoted to slightly different fields or characteristics of their fields, or worshipped by different groups. To what extent these aspects are separate beings, and to what extent they are aliters of a single being, may be debatable, and indeed may vary by strain of gods, or by individual god. Further complicating matters is the fact that an aspect of a god may sometimes eventually split off and become another god in its own right. Conversely, a formerly independent god may later come to be considered an aspect of another god. It's even possible that some people may come to consider one god an aspect of another god, while other worshippers continue to regard them as separate beings. What this implies about a god's individuality has been a subject of much debate among theologists.
Related to aspects are incarnations, or avatars, mortal forms that gods may take on for various reasons. Many gods choose never to deal with mortals or their worlds in person, instead sending only a temporary avatar to carry out their business, vastly powerful but much less so than the actual god. Typically, such an avatar remains connected mentally to the god, with the god fully aware of everything the avatar does and senses, but this is not always the case. Some gods, for reasons of their own, may occasionally choose to live a lifetime as a mortal, though given the gods' usual power of pluripresence this does not prevent them from simultaneously retaining their divine nature in another corpus.
Pantheons
Even on worlds where multiple gods exist, some gods are solitary, their faithful worshipping them alone and dismissing all other gods as false or evil. Many gods, however, exist as part of a group of gods called a pantheon. Pantheons may be related by blood, or they may simply be allied, or associated by a common pool of worshippers—different pantheons are often worshipped by different nations or ethnicities, or even by different races. Many mortals devote themselves to an entire pantheon rather than to a single god, and even a worshipper of one god of a pantheon often gives at least lip service to all other gods of the pantheon. A pantheon may have a single leader, sometimes called a thearch; this leader may be the literal parent of other major gods in the pantheon. Often there are also one or more gods who are frequently in opposition to the rest of the pantheon, but who are nonetheless considered part of the pantheon despite their adversarial relationship. These gods may even be considered evil by their fellows, but mortals may still honor them along with the "good" gods of the pantheon.
For the most part, if the gods of the panthon have fields, then each god will have a different field or combination of fields; if a new god joins or is born into a pantheon, they will find an empty role to fill. It is not impossible, however, for two gods in a pantheon to share a common field, though they generally represent different aspects or treatments of it. The leader of a pantheon, if they have any field at all, usually has a broad or prominent field with positive connotations.
Most gods belong to a single pantheon, but there are some gods who are worshipped on many worlds and incorporated into multiple pantheons, though not generally by the same cultures or worshippers. Such gods are sometimes known as universal gods, though this is something of a misnomer given that they are not in fact necessarily known and worshipped on all worlds of a given universe (nor, conversely, is their worship necessarily confined to a single universe, either). There is evidence that some universal gods are aliters of passivites, but this is unlikely to be the case for all of them.
Gods and Magic
Gods are, necessarily, magical beings. They are able to command considerable inherent powers without the aid of external devices or mechanisms; this, by definition, is magic. However, this is not to say that the god's followers, or indeed the god himself, will always concede that definition, or will look favorably upon other uses of magic. There are some gods who tolerate or even encourage the use of (other forms of) magic among their followers—there are even gods of magic for whom magic is an essential part of their worship and dominion—, but there are also those whose dogmata hold all other forms of magic to be false and evil, or who deny that any other form of magic exists. In fact, other forms of magic may not exist; it is entirely possible for there to be no magic in a particular world or cosmos apart from that directly attributable to the gods, though this is uncommon. It's also conceivable, for that matter, for it to be true that all magic aside from that of the gods is evil; it could be that the only form of magic aside from that of the gods is magic provided by nefandous demons who exact their own price for its use. Again, however, this is rare; on most worlds where gods exist, other forms of magic (not associated with demons) are also present, whether or not the gods choose to acknowledge or condone them.
It's common for devotees of gods—or on many worlds even for the populace in general—to use different words for the magic of the gods than they do for other magics. The powers of the gods may not be referred to as magic at all, but by different words entirely, such as miracles, or simply divine power. Even where it is acknowledged to be a form of magic, a semantic distinction may be made between the divine magic of the gods and the worldly magic of mortals. (The terminology, of course, may vary across different worlds.) This may or may not correspond to any qualitative differences between these forms of magic; even when the gods' powers are obviously similar in working to other magics, and even subject to the same metacelemics, they may still be called by different names.
Homes
Though mortals may refer to temples and churches as the dwelling places of the gods, this is not in general literally true. For one thing, not all gods have a home at all. While most gods have the power of pluripresence, some few are actually omnipresent, and therefore have no need to single out any particular location to call their homes. Even gods that do exist in physical locations may not have the need for a lair; able to create or summon to them on a whim anything they desire, they need no place to store goods, and having no requirement for food or sleep, they need no bed or dining hall to retire to.
Still, some gods do choose to designate one or more homes and centers of their power. They may want a place to keep or display powerful eximia, mortal or immortal servants, and objects of sentimental value that even with their divine powers are not easily replaced. A specific lair might serve as a focus for mortal worship, or might allow the gods to consolidate and increase their power. Even gods may have emotions and desires, and they may find it comforting to have a place they can specifically call their own. Some may go further, and actually hunger and tire like mortals, and do need a place for rest and refreshment after all. For these reasons and more, gods do often have residences—perhaps many—in which they may spend some time.
Some gods might indeed choose to make their homes in temples and churches, or elsewhere in the mortal world—gods of nature, for instance, might favor secluded glades or other natural areas. More often, however, gods dwell apart from mortals, either in distant places far above or below where mortals dwell, or, more likely, on other planes entirely. A god may have an entire plane to himself or herself, or gods of a pantheon—or gods of similar principles or fields across several pantheons—may have separate realms (or shared realms) within a single plane. A plane devoted principally to the residences of gods is called a divine plane, though gods may also make their homes in propositional planes related to their fields and doctrines or in the same eschatological planes to which they send mortal souls.
Servants
As powerful as gods are, they often have agents to take care of relatively minor matters for them. These servants may dwell with the gods in their homes, may lair on separate spiritual planes, or may be summoned by the gods when they are needed. While not nearly as powerful as the gods themselves, these divine servants are usually granted powers far above those of most mortals, and gifted as well with immortality. They may be elevated from faithful mortals—or, less often, unfaithful mortals bound unwillingly into servitude—, or they may be specifically created by the god, or coöpted from beings they find elsewhere, such as on some distant plane. Most of the time, the form of these divine servants either reflect the fields of the gods they serve, or resemble that of the mortal races that worship them, or both. This does not have to be the case, however, and their potential variety is endless.
Gods have their servants, too, among the mortals. Most obviously, there are the priests who officiate in their churches, but others may also serve the gods outside the clerical hierarchy. Many of these mortal servants of the gods may be granted special powers above those of worshippers who do not serve the gods directly, but even when they do not receive such immediate benefits they are generally promised great rewards in the afterlife.
The trust the gods place in their servants may lead them to exact dreadful consequences if it is betrayed. A servant of the gods who goes against its master's wishes or, worse yet, apostasizes from the faith entirely, is often subject to much more terrible fates than the gods mete out to other evildoers. Some gods reserve special punishments, ostentatious and horrifying, for any of their servants who so cross their masters.
Creation
A new god may arise in many different ways, though not all these ways are necessarily possible for a given strain of gods. Sometimes gods may procreate and bear children like mortals, leading to new generations of gods. Many pantheons are composed mostly or entirely of huge extended families. The birth of a new god is not always a happy occasion for the parents; some gods might fear their place being usurped by their offspring, and may take great pains to make sure that this doesn't happen—though these measures may not always be successful. Sometimes gods may procreate not only with other gods, but with mortals as well, leading perhaps to beings called demigods that are not quite god but more than mortal.
Procreation is by no means the only way that new gods can form. Sometimes gods are produced through a sort of spontaneous generation, coming into being in response to mortals' beliefs or desires. Sometimes, for whatever reason, mortals begin to honor and worship some object or mortal being, and this very veneration instills it with divinity and makes it into a new god. Sometimes the gods themselves choose to bring forth a new god, either directly using their godly powers of creation or by raising a mortal to godhood. The transformation of a mortal into a god is known as apotheosis.
A new god may already have its fields and powers set by something inherent in its formation. It may be, however, that the god must consciously choose its fields. In this case, the appearance of a new god may lead to much political maneuvering among the other gods, each seeking to guide the newcomer into a field that will support their own power blocs and interests.
Destruction
With perhaps some rare exceptions whose divinity some theologists might consider dubious, gods do not die of old age or of anything else that would typically be considered "natural causes". It may not be impossible, however, that a god could be killed. Given their vast power, such a deed would be monumentally difficult. While there have been accounts of gods slain by mortal hands, they are rare and controversial; probably only another god or perhaps a powerful eximium would have the ability to end a god's existence under normal circumstances, and even then it wouldn't be easy.
Impervious as they may be to physical violence, gods may in some cases suffer from more esoteric vulnerabilities. On some worlds, arcane diseases have been identified that can afflict gods, and perhaps even kill them. The lives of some gods may be bound up in physical objects that are well protected but not entirely invulnerable. And many gods require the worship of mortals; should all their worshipers desert them, such gods will find their divinity at an end.
If mortals have afterlives, then the "death" of a god may not be the end, either. Some deities, their godhood at an end, have become simple mortals, and been thereafter subject to the destinies other mortals share. In some cosmoi or chorodeses, there may be divine graveyards where dead gods end up, perhaps persisting for eternity in some semiconscious state. Even where the ultimate fates of dead gods are unknown, theologists speculate that their divine souls may still ascend to some unknown plane.