One who cares about personal appearance, shops at nice stores, and is generally more image-conscience then others. The term prep has been identified with bitches and sluts, which is a stereotype and is rarely true.
Originally derived from preparatory schools, prep has turned into a term people use to classify a certain group of people who act and dress a certain way. This does not mean that all preps are the same or that they are all bad or good.
Prep seems to be used as a derogatory term, when really it is just another name for a clique or group of people. I think of prep as either a meaningless term or a positive one.
Person One: Did you see that slut face bitch? She is such a prep!
Person Two: Maybe if you took the time to get to know her you wouldn't think of preps in such a negative way :)