Deities are primordial forces in the cosmos. They are the puppet masters that all mortals jump to. Their abilities can not be measured in any rational manner, unlike demons or angels. They can be ranked against each other, but even so those rankings are based upon mortal perceptions and may be grossly inaccurate.
The deities exist in the Planes of the Gods. This is a loose term for region of the multiverse beyond the Astral Plane that is not the Abyss and is not any of the Planes of Men. These planes, in many ways, are extensions of their inhabitants and creators. The physical laws on each of these planes is dictated solely by the will of the deity or deities inhabiting it. Thus, they vary greatly amongst themselves, and several even vary from whim to whim of the local deities. There is no guarantee that any of the tenets of magic, animus, or anything else apply on any of these planes.
While many large regions of these planes have been set aside for the followers (deceased and otherwise) of various deities, other regions are completely hostile to mortal life. Normal, unprotected animus cannot survive in many of these regions: the magical and animatic forces are simply too great.
Many religions try to place the deities in their pantheons in specific hierarchies. In general, this system rates the deities of a specific pantheon by a class system, similar to avatars and others. The important thing to remember, however, is that this is not the same level system used for demons or avatars. Deities have no levels measurable in mortal terms. This is, further, a completely relative rating of deities within a pantheon. A class VI deity in a minor pantheon, in objective power, may only be able to shape the course of time as much as a class II deity in a major pantheon.
In general, the power of a particular pantheon, and its ability to control events on the planes of men, is usually (but not always) commensurate with the number of worshippers on that plane. Deities within pantheons are usually very busy struggling between themselves for domination and usually will not pay too much attention to things beyond the realms of their worshippers on the planes of men. Thus, most friction between deities comes from within a pantheon.
Pantheons, however, do not live in a vacuum. There are quite obviously other pantheons competing for control of the same nearly infinite number of (in)finite planes of men. One might presume that there is enough space to go around, but apparently not. In addition to rivalries within pantheons, there can also be rivalry between pantheons.
It is only within these absolutely horrifying time periods of struggle that one can make judgements on the powers of specific deities relative to those in other pantheons. Even so, it is nearly impossible to do, since so much of the battle is truly beyond mortal ken. Further, such battles are also usually indirect, played out by mortals like a game of chess (in the same manner deities in a pantheon struggle, only on a more epic scale). Often times, such titanic struggles may destroy entire nations (and/or banish/slay pantheons) or even entire planes.
Even in non-epic struggles, in everyday business, few deities get too involved. Except for certain very independent-minded deities, very few will ever engage directly with any opponent, or even work directly on the planes of men. Almost all prefer to work through mortal agents, with free will providing the randomness that makes the game exciting for the deities.
Agents of the gods, those magical beings variously called Saints, Angels, Avatars along with mortal agents used by prophecy or chance are for our purposes simply referred to as Avatars.