FRAPS and programs like it show you how many FPS are being demanded by the game engine, which changes dynamically based on the resources available. To an extent it also measures what is provided, but recently the discovery of "runt" frames has messed that up a fair bit.
Frame-rating is fairly complex thing to understand
Skip to 0:30:00 on this video, where there is a quick explanation of what Framerating is.http://www.youtube.com/watch?v=noVu8BXkCsw
Everything after that is worth listening too, the interview goes for about half an hour.