FRAPS is very simple - you just run the program, and it sits in your system tray. Whenever you load up a games, the FPS will show. You can also benchmark your FPS as well, although I'm not sure how to do it. FRAPS also allows you to record gameplay - and take screenshots as well. Recording gameplay does stress the CPU, and can drop your frame rates quite a bit, especially if you have a single/dual core. It displays your FPS in the top or bottom corners of the screen, you can choose where it displays it.
FRAPS is shareware and costs $37 - a demo version is available for free which is all you need to check your FPS, and can record gameplay for 30 seconds with a watermark though, and can only take images in .bmp format.
its home page then answers your questions, its a 3rd part app that runs in the background. You can select it to display your FPS in any corner of your screen. You can also set it up to run min/max time runs to benchmark various set ups. It will also capture video.