In legend, a familiar or familiar spirit is a supernatural being that helps and supports a witch or magician. Traditionally, a familiar is an animal, but some are said to be humanoid. Familiars often have special powers of their own. When witchcraft is portrayed as a type of communication or alliance with evil forces in order to gain magical powers, a familiar may be considered a type of demon.
The stereotypical familiar in Western culture is the black cat. Because black cats are strongly associated with witchcraft, there are a number of superstitions regarding them. A black cat crossing ones path (in the US) is said to be a portent of doom, for example; in the UK it is considered to be good luck. A familiar may be any animal or spirit guide. Other common forms for a familiar to take are a dog, an owl, spider, and toad.
In the days of widespread persecution of witches, every witch was believed to have a familiar, and close animal companions were sometimes considered proof that a person was a witch. In addition to animals, humanoid creatures were believed to serve as familiars; sometimes, these familiars looked like regular people, and sometimes, they were said to be odd in appearance, having some deformity or resembling stereotypical images of demons. Legends of this period also often attribute the habit of drinking blood to familiars. Familiars were considered at least as dangerous as witches, as they were thought to be supernatural beings that looked like normal animals and could spy or wreak havoc for their witch without being easily detected.
While Christians traditionally interpret familiars as demons, to Wiccans and Neo-Pagans, they are more like the Christian concept of a guardian angel. A witch's familiar can be his or her closest companion, offering moral support, special knowledge, and/or physical healing. Witches may seek a familiar through meditation or divination, but most do not believe that familiars can be summoned, contrary to traditional Western legend regarding witches and black magic. You can of course petition your familiar to make itself known to you through ritual or prayer - be sure to keep a close eye out, as familiars come in all shapes and sizes.