Lupin, in case you didn't know, is a variation on the latin Lupus meaning wolf. Knowing that, I suppose it would really be no big surprise that he is, in fact, well... he happens to be a... a... a... (*Whispers*) Defence Against the Dark Arts Teacher. (*Sigh*).
Oh yeah, he's also a werewolf - but that's really rather secondary, don't you think?
Ahem. Anyway, Lupin is billed by Harry as the best DADA teacher he's ever had - which is probably true considering the competition is Crouch (a.k.a Moody) (evil), Lockhart (incompetent), and Quirrel (both). He's still a nice guy, though. And Snape doesn't like him.
His alias is Moony, and the thing he fears most is the full moon (not, in fact, the crystal ball as Lavendar and Parvarti seemed to think). Snape brews a potion for him that stops him losing control when he becomes a werewolf, though, so he's mostly safe. He loses it when he forgets to take the potion when Sirius and Harry are carting Peter Pettigrew to Hogwarts, though, and as a result Pettigrew escapes. He disappears after that, quitting his job after Snape announces to the world his werewolfship. *sniff sniff* *sobsob* But he'll return in the fifth book. (^.^)