There are four kinds of familiars. The first kind is a physical creature who lives with and has a rapport with a human. These are usually called "pets." The "pets" who are truly familiars are the ones who establish a psychic link with the human of their choice. These physical familiars can communicate their needs without vocalizing. They are usually very good at telepathy.
The second kind of familiar is an astral creature of animal form which attaches itself to a human for the purpose of aiding her or him. This type of familiar may be attracted by an intense liking for a specific creature that one cannot actually possess in his or her physical life. The familiar may also appear because of a specific need within the human's life. Deceased pets often return in this capacity.
The third category of familiar is an elemental spirit. Ceremonial magicians and witches often call upon an elemental to inhabit an object, such as a talisman, statue, crystal, magick mirror or pieceof jewelry. The genie of the lamp or ring falls into this category. Paracelsus had such a familiar which lived in a large precious stonein the pommel of his ritual sword.
The fourth familiar is a discarnate human being or the spirit of someone who has died. Although a few practitioners of the art will command the appearance of a discarnate being, I'm not in favor of ordering such beings around. A discarnate spirit who has been commanded against his or her desires can be troublesome. Then you are faced with the necessity of getting rid of them, which can also be more difficult than the original calling. Discarnate spirits who are willing take the positions of astral teachers or guides, some people call them guardian angels.
From Animal Magick by DJ Conway