The Werewolf Legend
According to the old tales, a werewolf is a human who turns into a wolf at full moon. To become a werewolf, one had to be bitten or scratched by one, or be placed under a curse. The werewolf legends are mostly European, although Native American legends do recount men transforming into animals, called shape-shifters.
The wolves in Forks are Native American shape-shifters, not traditional werewolves. In the Twilight universe, the latter do exist and are mentioned later in the series but are never fully introduced.
The Twilight shape-shifters transform in order to protect their area from vampires. Theirs is a genetic rather than a magical transformation.