You’ve already heard about the MacOS X Exposé functionality. This is where you can activate the display of either all open program windows, all windows of the active program or the desktop. You can even activate the screen saver or hibernate mode of the display. All these functions can be activated by function key or by moving the mouse pointer into a so called “active corner”.
During normal work you may enter the active corners accidently from time to time. This is a little annoying but the ingenious Apple engineers have already implemented a cure for this itch. You can add modifier keys to the active corners. To trigger the active corners function you now have to press the modifier key while moving the mouse pointer into the corner. No more accidently activation from now one. You have even assign different modifier keys to different corners. But you can’t overload an active corner with multiple modifier keys
How it works show this little screencast. Although the audio is in german you should get the idea how the modifier keys can be set in the system preferences.