Exposé is almost unexplainable. It's an incredible time-saver. It is controlled by the F9, F10, and F11 keys. Essentially, it 'tiles' (for lack of a less windows-y word) all the windows you have open, and then you click on the one you want to bring forward (F9). F10 does the same, but only with the application you're currently running (i.e. all the Safari windows). F11 whips all the windows to the side of the desktop, so you can get at your desktop icons.
The only way to get the hang of it is to try it out. Open a bunch of windows, and try them.
I've been using Mac's for a few years but only upgraded to OS X 10.3 with my new iMac so have only just started with exposé.
Like rand() says it allows you to shift all open windows either aside from the desktop of tiles them for easy switching between them and it works a treat.
I'd read about and it never sounded as good as it is.
Advices though, get a multi button mouse to use it to its full potential. I use a MS Bluetooth mouse and it works an absolute dream with the side mounted thumb buttons instead of the F-keys.