An Avatar is the physical host that a God inhabits when personally visiting the Material Plane (as Gods themselves do not occupy physical space). Gods typically select their most faithful followers, which lends to the theory that the vessels power is related to it's devotion.
A God is able to completely take over most mortal vessels without that mortal's consent, but that body's power would be greatly compromised. A God can also share the body with the mortal, creating a mortal god with access to all of that mortals knowledge and memories.
This being is equivalent to the ideal, perfect specimen of the host's race; with access to 100% of it's physical, intellectual and magical potential at it's disposal. They are also expert spell casters, comprehend all languages, and are proficient with all weapons and armor. However, they are still mortal.
If slain while in this state, the God's consciousness will be rendered dormant. While dormant, a God is unable to act on it's own in any way, they but can be re-awakened by a cleric of sufficient talent with relatively little effort.