Exposé is a special feature of the Mac operating system that was first introduced with Mac OS X 10.3. Exposé lets you view all open Finder windows or all open windows in a single application at one time so that you can bring one easily to the front. The best way to understand Exposé is to view a screenshot of it in action.
As you can see in the screenshot, all of the open
windows on my desktop can be viewed at one time. To
select one window, I would only need to click on it and
that window would become the active, frontmost
window.
You have three options for setting shortcuts for
Exposé on the Exposé preferences pane in the
Mac OS X System Preferences. You can set keyboard or
mouse shortcuts to open:

To set keyboard and mouse shortcuts for Exposé, click the buttons next to "All windows," "Application windows" and "Show Desktop" and select the shortcuts you want from the drop-down menus.