A familiar is the companion of a witch, who aids in the practice of Eastern witchcraft. In most cases, this is an animal of some kind, although there are exceptions to this. A familiar forms a close psionic bond with their witch, and is able to communicate with them.
Witches draw energy from themselves (not, as a sorceror does, from an external source) to perform spells. A familiar is therefore vitally important for several functions including helping the witch focus, contributing energy to spells, protecting the witch from outside threats especially when he is vulnerable during and after spellcasting, and warning the witch when his energy is dangerously low.
In some cases, a witch may acquire more than one familiar. (For example, the Warlock Lord Brimford (aka Lászlo) has two familiars.) In other cases, a witch's familiar will not be an animal at all. The Great Weapon Blackwand, for example, serves as Morrolan's familiar.