Werewolf is a creature type.
They uniquely uses both sides of the card.
Although other creature types have Double-faced cards, none are exclusively two sided as are werewolves.
To play a Werewolf card, cast it for its Mana Cost on its original side. It will have a Triggered Ability that turns its other side face-up, changing the creature in almost every aspect: in Art, Rules Text, and Power/Toughness.