Deity

Deities, also known as gods, are powerful, divine beings. Little is known about them for certain, but it is known that every deity has within them a divine spark that is the source of their powers. While the origins of most deities are unknown, there are a very small number of deities who are known to have come by their divine sparks in modern times, through various means: Kiaransalee was a mortal raised to godhood by Lolth by splitting her divine spark, while Boccob granted his divine spark to Ioun at the moment of his apparent death. So far, Vecna is the only known instance of a mortal becoming a deity seemingly under his own power, but as he is the god of secrets it is unlikely the knowledge of how he accomplished this feat will ever be spread.

Though the divine spark makes the bearer functionally immortal, it is possible - though extraordinarily difficult, and perhaps impossible without being a god oneself - to kill a deity. This was never evidenced more clearly than in the Deity War, in which Nerull and those allied with him killed a large number of deities in order that the Reaper of Souls might harvest their divine sparks. It is unknown what happened to the sparks Nerull had collected when he was finally destroyed, but most scholars believe that they likely dissipated back into the universe (the more optimistic ones believe that they will one day be used in the creation of new deities to replace those who fell, often citing the Raven Queen as a theoretical example), though the Del'Braeth clergy believe he found one in his explorations of Syra.

Avatars
An avatar is, essentially, a less-powerful duplicate of a god. An avatar serves as its god’s alter ego, effectively allowing the god to be in several places at once. They are not a separate being so much as an extension of their god, and as such cannot act independently of their creators.

Aspects
An aspect is a powerful servant of a god, created in that god’s image. As opposed to avatars, aspects are distinct individuals possessed of independent thoughts and actions, though they wield considerably less power than the gods themselves. Aspects can in many respects be thought of as more powerful angels, being created in much the same manner and sharing many of the same traits.