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