Often referring to yaoi manga, ukes are usually the bottom ones. Opposite of semes, ukes are usually cute, has less experience with sex, kind, those kinds of things.
The original meaning of uke is one of two training partners in martial arts. The tori is the partner performing a technique, and the uke is the partner receiving the technique.
The person on "bottom" or the submissive person during intercourse. Contrary to popular belief, this word is not always associated with sonen-ai or the likes thereof.