An animal used in
spellcasting, often thought to be more than just an animal but a
spirit in animal form.
However familiars are more than just a tool for magick. A familiar is the spellcaster's companion and in many cases friend. The two share a mental bond, frequently of almost telepathic level. The two share a life-bond, a symbiosis as it were; there is a mutual agreement between them. The arrangement is *not* the spellcaster just *using* the animal, but the creature agrees to lend the human his/her powers when needed, in return for life-long protection and care. Except in incredibly incredibly unusual circumstances, familiars are taken *one* at a time because of this life-bond. To "disown" your familiar by taking another is the highest level of betrayal imaginable.
Familiars must be animals, one spellcaster cannot take another human as a 'familiar'--*will* gets in the way; the presence of sentience in some odd way negates the ability to be a familiar. This is *not* to say that the familiar animals are stupid or unthinking; rather they think and act on an emotional/intuitive/instinctive level rather than a cognitive one.
Common familiars are cats, owls, and (if you believe in them) small dragons. Also taken not infrequently are rats, bats, and toads. There are one or two recorded cases of skunks as familiars as well. Familiars are not *limited* to these animals; rather these species tend naturally more towards psychic planes and so are able to provide the needed talents, but any animal *can* have familiar potential and any species can have its representative.