i've heard different things so i guess you should research and decide for yourself. historically [as Lark pointed out] the familiar was the pet [usually a black animal like a raven, dog, or cat] of some 'creepy old lady'. something bad happens, someone notices the womans pet suddenly it was her fault.
i've heard familiars are animals you have a sacred bond with, animals you put extra energy into, your own energy [not physical] in the shape of an animal, or an animal who can help with magick and/or sense energy. [personally i believe the spiritual bond theory.]
i did read in Alaric Albertssons book To Walk A Pagan Path you could train you pet to join you in circle, meditations, spells and other magick workings [depending on the kind determines what extend of course] if you have a pet you could try bringing it into your practice. on the other hand, Deborah Blake [an author i love] mentioned how she has a couple cats and only one is understanding of circle [the others need to be put behind closed doors during coven meetings] so again, it depends on the animal. there is a bond i feel you should have, so observe you pet, understand sometimes you want to meditate but they want to play.