Smaller creatures that serve to aid certain spell casters. They typically lack the combat prowess of an animal companion, but aid a spell caster in other ways.
Familiars are the typical assistant to a spellcaster by either providing knowledge, aid in or with spells, or otherwise being a trusted companion (albeit typically not one skilled in direct frontal combat). Their jobs may vary from game system to game system but generally uphold the following:
Associated with spell casters
Subservient to its master
Have some form of communication with its master and possibly a more limited means with outsider entities
Familiars are distinct from animal companions. Questions about animal companions should use the animal-companions tag instead.