A person who can transform into a wolf. The scientific term for this -- which describes both the mythological monster and the psychological defect -- is lycanthropy, and the werewolf is called a lycanthrope or, more rarely, a lycanthropist. In myth and legend, the cause was often black magic -- the werewolf made a deal with the devil for the ability to change into a monster and eat people. But sometimes, the werewolf was the victim of a curse or was even a good person who used his ability to do good deeds.
Werewolf myths and legends are present in some form in almost every culture in the world, from the loup garou in France to the fox people of China and Japan to the boudas, or hyena people, of Morocco to the santu sakai of Malaysia to the Nagas, or snake people, of India to the skinwalkers of the Navajo...
In many tribal cultures, where the wolf was respected as a hunter, the wolf was emulated or even worshipped, and hunters sought to take on the characteristics of a wolf so they could bring food back for the tribe. Later, as humanity became a more agricultural species, the wolf became an enemy, and the Church decreed that werewolves were in league with Satan. A number of people in the Dark Ages were executed for being werewolves, but eventually, even the Catholic Church realized that lycanthropy was a mental illness and began recommending that no more werewolf executions take place.
Nowadays, the werewolf is most visible as the star of various movies, including "The Wolf Man," "I Was a Teenage Werewolf," "An American Werewolf in London," "Ginger Snaps," and many others. Werewolves also pop up in books, TV shows, video games, comic books, roleplaying games, and even songs (most famously in Warren Zevon's "Werewolves of London"). There is also a small subculture of werewolf fans/new agers/neo-tribalists who claim that they can psychically change into werewolves -- though since it all takes place on the astral plane, it's kinda tough to confirm their claims.
Symbolically, the werewolf usually represents the struggle between man's civilized nature and his base, animal side. Every person, no matter how good, humane, and cultured he or she may be, harbors the occasional desire to raise hell, to live the wild life, to throw off the shackles of civilization, howl at the moon, and tear our co-workers' throats out. But who will win out -- the man or the beast...?