Officially, Giles is the librarian at Sunnydale High School. Unofficially, he's only in Sunnydale to make sure Buffy is as well-trained and well-informed as possible in her fight against evil. Giles comes from a long line of Watchers, and it's his duty to provide guidance for the Slayer -- without getting personally involved in her battles.
Typical quote: "We're at the center of a mystical convergence here. We, in fact, stand between the earth and its total destruction."