Familiars
A Familiar is a very particular type of Divine Vassal - a type of quasi-divine creature that originates on a realm ruled by a deity. They fall in the category of Divine Vassals although many also act as Proxies if they are affiliated to a patron divine power.
Appearance
As Familiars are also proxies they can appear in any insect, bird, reptile, mammal, or amphibian form native to their home plane. Again, as with proxies, they may be assigned a different form when sent to the Prime Material Plane. All familiars revert to their "true" form when on their home plane.
Humanoid Familiars
Some familiars retain the ability to return to their "true" form when on the Prime Material Plane. Notable familiars of this kind are Imps, Quasits, Baltir and the like, who all have the form of a small humanoid.
Purpose
All familiars have a specific purpose, which is to provide aid and advice to the servant of the Deity to which they are assigned. Without a specific mortal master, these creatures are "merely" proxies. And without an affiliation to a divine power they are just divine vassals.