One of the features in OS X is the Mission Control window management system, which will show you various views of current windows for a single application or all applications. The view for all windows for a single application is called "Expose," and as with other Mission Control features it should smoothly toggle on and off when triggered. However, there may be an odd occurrence or two where this view may get stuck and not shut off.
When this happens, it may appear that no application (including the Finder) will reactivate. While applications like iTunes may continue to function in the background, the Expose view will cover everything, and prevent you from using the mouse to interact with any of these programs.
If this happens, then you might thing that forcing the system to restart is the only option, but this is not the case, and there is a relatively easy way to clear the problem without interrupting your workflow.
Fixing the problem is a matter of realizing that Mission Control is governed by the Dock in OS X, so if a problem occurs with it, then you can force-quit the Dock and clear the problem. Since the Dock is a persistent service in OS X, it will automatically re-launch when you force-quit it, and be up and running again.
To do this, you will need to launch the Terminal to run a command; however, since you cannot use the mouse to open the Terminal, you will need to use the Spotlight menu. Therefore, press Command-Space to open the menu, and type in the word "Terminal," or for more specificity, type "Terminal kind:Application" and you should see the Terminal appear as the Top Hit option in the menu. Now press Enter to launch the Terminal, followed by running the following command (simply type it, including appropriate capitalization, and press Enter):
When done, the stuck Expose view should disappear and the Dock should then reappear from the bottom or side of your display (depending on how you have it configured), and you should be up and running again.