Werewolves (also known as Lycanthropes) are a catagory of fantastic creature.
Werewolves are a catagory of shapeshifters that can change between a 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). Said transformation is usually involuntary and based in some way around the cylces 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 transformational abilities. Werewolves have a vunerability to silver as well as the plant Wolfsbane. Being bitten or 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 means and some beleive that the myth itself has its origins in pre-industrial people dealing with individuals infected with diseases such as rabies.