Just like earlier versions of OS X, Lion and Mountain Lion lets users see all of their Mac's open windows with the press of a button. Mission Control groups all of those windows by application, but some long-time Mac users prefer the old show-ever-open-window-in-a-grid style view instead. Long-time users of OS X may remember when Exposé was first shown off in OS X 10.3 Panther. OS X had proved to be great at running many apps, but with the new ability, a new problem emerged: With many apps comes many windows, and an ever-increasing inability to find just the one you're after.
Exposé was Apple's way to eliminate the issue, and when Steve Jobs demoed it at WWDC 2003, people cheered. This is because Exposé shrunk all of your windows down so you could easily find the one you were looking for, and it was really cool.
However, the feature changed starting in OS X Lion and many were unhappy. Exposé morphed into Mission Control, and instead of showing you every single window in a way that made them super easy to find and select, windows were now grouped by application. This wasn't a big deal if you only had one window open in the app you were after, but if you had more than one open it wasn't nearly as useful.
Mission Control with grouped windows
Thankfully in Mountain Lion, Apple introduced an option which made me cheer aloud when I found it. You can now restore the original functionality of Exposé which shows every window, ungrouped, and easy to find.
Mission Control without window grouping
To bring back the old behavior, head into System Preferences > Mission Control, and uncheck "Group windows by application." After that, you'll be able to see every window, ungrouped, in Mission Control. If you prefer the new behavior, simply go back and re-check the box. Enjoy!