A derogatory term, generally targeted at men. By connotation, this word carries with it an accusation of being a sexual predator of one sort or another.
If a man is called this by a woman, it is usually a very serious insult, and not one uttered lightly. As evidence, note that a man calling himself a creep is likely to be met with swift (and often surprisingly vehement) assertions to the contrary by any female friends within earshot, regardless of any reason the man might have for calling himself such.
The use of this insult is in something of a decline in recent years, in favor of more specific terms such as stalker. However, this seems to add to the force behind it when it is applied.