A Young student in high school or college who:
1. Performs well academically
2. Plays sports such as Crew, Tennis, Golf, or Lacrosse
3. Comes from an established "Old money" background
4. Is favoured by their school, church, and other local establishments
5. Is good looking
6. Went to/ goes to preparatory school
Of course, a prep need not have ALL these qualities but many do. A common misconception about preps is that they consist only of the Patrician New England "Old stock" establishment. While this is generally true to a large extent this is not to exclude the types of preps who come from the Southern and Southwestern regions of the US (who more closely resemble the "popular" definition of preps but we'll get into that in a minute). Students who fit the definition of "preppy" can be found in any part of the world, including the upper circles of Europe, and Latin America.
Preps have historically been White Anglo-Saxon Protestant, however, this criteria has somewhat changed in recent decades of racial/ethnic integration.
Preps represent all sides of the political spectrum, the more liberal ones being found in Connecticut or Massachusetts, or the more conservative ones being found in Georgia or Texas.
Preps vary in their attitude towards others and range from being completely condescening to being all inclusive. The majority of preps are fairly indifferent to those outside of their own type, usually not out of snobbery, but out of their tendency to be more private as individuals.
Perhaps the biggest debate involving preps today is the definition of "prephood". The New England preps often state the the more common definition of prep defined by people who wear Hollister, Abercrombie & Fitch, etc. is a false construction, whereas the "true prep" actually wears brands like J Crew, LL Bean, Eddie Bauer, and Ralph Lauren. Whlie a true prep is more inclined to match the latter definition, there is not a rigid line between types and many preps actually wear a mixture of the two styles, wearing the established brands for school or church and the newer brands in casual settings. Of course, it is important to note that true prephood is NEVER defined by clothes alone.
Finally, preps are often seen as conformists, but in reality, this perception comes from their emphasis on constructing highly esteemed careers and willingness to accept the concept of the "establishment" as an inevitability. Thus many preps aspire to go into business, medicine or politics later in life.
I pretty much match the definition of a prep and am pretty proud of it. =)