From the Wikipedia entry on Familiars:
In European folklore and folk-belief of the Mediaeval and Early Modern periods, familiar spirits, sometimes referred to simply as familiars, were supernatural entities that were 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]”.
Due to their association with older forms of magic, in the twentieth century a number of magical practitioners, including adherents of the Neopagan religion of Wicca, have once more begun to utilise the concept.
When they served witches, they were often thought to be malevolent, whilst 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 being demons, whilst the latter were more commonly thought of and described as being fairies.The main purpose of familiars is to serve the witch/young witch. The service the familiar would provide would be to protect the new witch coming into his/her new powers.
The most astute comment that has been made to me about smartphones in the last year was by my friend Colby Miller up in Vermont over christmas. We were talking about our android devices and while holding out his HTC Incredible he said “you know what this is, it’s a digital familiar“. I had never thought of it in those terms, but it made perfect sense, and really captures the kind of relationships we form with these devices.
Familiar isn’t apparently part of most people’s vocabulary, as I found out last night. I guess all that AD&D in middle school came in useful after all.