Hi,
Actually the Wiki definition referenced earlier has a lot of good information about archtops. I would have to disagree about classifying a Les Paul or 335 as an archtop, even though they have an arched shape to their top.
Usually archtop not only refers to the shape of the guitar's top but also how the body is contructed. The classic archtop style guitars (like a Gibson L5 or Benedetto) usually have a completely hollow body, and often use a solid piece of carved Spruce for the top (as compared to a laminated top which is less resonant). These guitars can be used acoustically, but most people rely on pickups because they're not typically loud enough on their own. Archtops are usually very expensive guitars, if you get a decent one, because of the quality of contruction required and quality of wood, etc.
If you want to hear the sound of an archtop just check out Wes Montgomery. He used a Gibson L5 which is one of the classic archtops.
-- Jim