The worgen are evil wolf-like humanoids that came from a dimension of pain, where no corner is safe. They were accidently brought into Azeroth by s night elf with the Scythe of Elune, and later taken control of by a human mage named Arugal. When the worgen turned on him and slaughtered the humans he was trying to protect, he lost his mind. He adopted the worgen as his children.
Worgen are not werewolves. Being bit by one won't turn you into a worgen, though being cursed by Arugal might.