Anime: Fullmetal Alchemist
Manga:
Fullmetal Alchemist
Spoilers are coloured, naturally. The manga and anime differ at varying amounts at times, but Lust seems pretty consistent.
Lust, named after one of the seven deadly sins, is one of the real bad guys in FMA. She, along with the other sins (excepting Greed) are seeking the Philosopher's Stone, a stone possessing tremendous amounts of power, with which they can become human. Whether they have another plan for the stone besides that, it has yet to become clear (world domination wouldn't surprise me).
It's obvious from early on that Lust and the other sins are not quite human. Lust is actually a homunculus, an artificially created human being. They are created from failed human transmutation experiments involving resurrecting the dead. It's not been explicitly said yet, but it's become clear that Lust was originally Scar's brother's girlfriend before she died, which makes Lust one of the youngest sins.
Unlike some of the other sins (Greed and Gluttony in particular), Lust doesn't seem to especially act in accordance with her sin. She is a fanservice character though. She attacks by extending her fingers, and her homunculus tattoo is on her chest.