Werewolves (also known as Lycanthropes) are a category of fantastic creature.
Werewolves are shapeshifters that can change between human and canine form. Said canine form is generally either a Grey Wolf or a semi-humanoid bipedal creature with a large number of lupine charecteristics (fur, muzzle, and so forth). The transformation is usually involuntary and based in some way around the cycles of the moon (generally this is related to the full moon, either happening at night when the moon is full or triggered by seeing the full moon) although a few types of werewolves have more exact control over their transforms.
In pop culture, werewolves have a vulnerability to silver as well as the plant wolfsbane. Being bitten (and in some tales, clawed) by werewolves can cause other normal humans to become werewolves themselves. Many aspects of werewolves vary from depiction to depiction.
In some more modern depictions, lycanthropy is caused by a mutagenic virus rather than by supernatural forces, and some believe that the myth itself has its origins in pre-industrial people dealing with individuals infected with diseases such as rabies.