A god is essentially an extremely powerful creature tied to their home planet. They are in some ways like mortals, in that they can die and have corporeal bodies, but in some ways unlike mortals in that they hold powers to reshape reality.
There are multiple worlds, and if you look up at the night sky you can see them. The many different gods people worship all inhabit and rule their own world. It is unknown how, but being a god means inherently to be tied to that world somehow. Gods rarely ever leave their homeworld and none have done so for 900 years.
In exchange for worship a god grants the ability to speak his or her language, this usually requires intense study and any slight misprononcuation can have disasterous results. When performed correctly however a priest may be able to create miracles.
These are races created or molded by a god to serve them. Humans, being decended from lowly apes over thousands of years are one of the few intelligent non-servitor races. This is why they have five fingers on each hand.