A lot of people want to know what a familiar is.
In the old days familiars were believed to be demons that take the form of an animal to help witches but witches were also believed to be ugly and ride brooms so we don't know if that's true.
Through time as people got more informed about witches they learned that familiars could be a pet that was or is very close too you or a spiritual animal that guides and/or helps you.
It's believed that a witches familiar if not a pet but a spirit or demon it tends to take the form of a cat.
I did some research and found this
'In European folklore and folk-belief of the Medieval and Early Modern periods, familiar spirits (sometimes referred to simply as "familiars" or "animal guides") were supernatural entities believed to assist witches and cunning folk in their practice of magic. According to the records of the time, they would appear in numerous guises, often as an animal, but also at times as a human or humanoid figure, and were described as "clearly defined, three-dimensional forms, vivid with color and animated with movement and sound" by those alleging to have come into contact with them, unlike later descriptions of ghosts with their "smoky, undefined form [s] ".
When they served witches, they were often thought to be malevolent, while when working for cunning-folk they were often thought of as benevolent (although there was some ambiguity in both cases). The former were often categorized as demons, while the latter were more commonly thought of and described as fairies. The main purpose of familiars is to serve the witch or young witch, providing protection for him/her as they come into their new powers.
Since the 20th century a number of magical practitioners, including adherents of the Neopagan religion of Wicca, have begun to utilize the concept of familiars, due to their association with older forms of magic. These contemporary practitioners utilize pets, wildlife or believe that invisible spirit versions of familiars act as magical aids'
Familiars are like little helpers to some there pets that was turned into the persons familiar with a spell (the animal still looks the same)
To other people a familiar is a demon or creatur that you can summon and after earning it's trust it's your familiar
The idea that a familiar is a physical animal or a pet is based on the accusations of witchcraft in the Middle Ages. In those days the most likely person to be accused of witchcraft was some older woman living alone, likely with a cat or two for company. If she talked to her pets that was considered a sign of witchcraft. That bit of nonsense has been perpetuated by some of the crop of poorly researched books on the market these days. But the fact of the matter is that our beloved pets and a Witch's familiar are very, very different things.
A Witch's familiar is an animal spirit or a magical construct created by the Witch or the Magician as an adjunct to their magical workings, as a repository for power, and if needed as a defense against magical attack from another magic user. A Witch or a magician can draw upon the energies of the familiar when they need an extra boost of energy in a magical working. Conceivably if the spell is important enough, the Witch could completely drain the familiar of energy causing it to "die" or cease to exist. And in the case of a magical attack, the attack could be diverted to the familiar so that it would be the familiar which is destroyed rather than the magic user. None of these are things that I think any of us would want to do with our pets.
There is an excellent article on familars here: http://en.wikipedia.org/wiki/Familiar_spirit
Animals are very sensitive to the energies around them and they may be drawn to us when we are working magic because they can sense the energy. They also want to be around their people, which again will make them want to be where we are..and that may be where we are doing magic. Neither of these things makes the animal a familiar or inherently magical in and of itself.