In Mac OS X Lion, Expose merged with Spaces and became Mission Control. When you tapped the default F4 key on your laptop (or F9 or use a three fingered swipe up on your trackpad) to launch Mission Control, you’d get the image on the top left in the screenshot above: all the windows of un-hidden open apps at once.
OS X Lion changed things up by grouping all the windows from each app together in Mission Control, like the image in the lower left corner of the above screenshot. This new style, continued in Mountain Lion is intended to be an easier way to find the specific window you’re using. If that doesn’t work for you, you’re not out of luck, provided you’re running the latest big cat OS.
If you want to go back to the Exposé-style way of showing all the windows open without grouping them by application, head to the System Preferences app, which is found in the applications folder, or your dock or apple menu. Launch it, and then click on the Mission Control icon to bring up the Mission Control preference pane.
If you want to group all your windows by app, leave the “Group windows by application” checkbox ticked. If you want to ungroup them, freeing them from the tyranny of app-association, for example, simply un-tick the checkbox. It’s a simple, yet hidden preference, but should help you make your Mac work just a little more the way you want it to.
Via: OS X Daily