Familiars are a specific type of follower of supernatural creatures. The most common type of familiar would be a witch's black cat. 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 colour 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 categorised 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.
A familiar spirit can also be depicted as an alter ego, doppelgänger, personal demon, personal totem or spirit companion, and may be the double or the alter-ego of an individual. It does not look like the individual concerned. Even though it may have an independent life of its own, it remains closely linked to the individual.
Since the 20th century a number of magical practitioners, including adherents of the Neopagan religion of Wicca, have begun to utilise the concept of familiars, due to their association with older forms of magic.
The easiest way to understand the form of a familiar would be in Harry Potter and the Prisoner of Azkaban. The Patronus Charm (Expecto Patronum) which is used by all of the characters to defend themselves against Dementors. They are simply an evokation of positive energy which is represented in the form of an animal, and therefore seems similar to that of a familiar, such as Harry's stag.
Another way to put it would be Sabrina's cat Salem in Sabrina the teenage Witch - an American sitcom based on the Archie comic book series of the same name. The show premiered on September 27, 1996, on ABC all over the world.
