Intelligent, Sophisticated, Stylish, Often Popular; Sometimes Well-Off, but not always, as thought in the general stereotype. Abercombie and Fitch, American Eagle, Gap, etc. are not expensive, as some people think. The aforementioned brands are not really significantly stylish, but are of good quality, nevertheless. And not all preps are stuck up, as thought in the general stereotype. There are variations of preps, just as there are of any other social category of people.
Go to school, the mall, etc., see for yourself.