Yoda-may-the-force-quit-with-you
There are a couple of ways to force an app to quit, which can be useful when one is frozen, or it just decided to launch itself and you don't want it to (like iTunes does when you plug an iDevice in):

  • aim for the kill: right clic on the dock icon. If it shows "Quit" instead of "Force Quit", press the Alt key and it should toggle to "Force Quit" 
  • the worm in the Apple: clic on the Apple icon (top left of your screen), select "Force Quit", pick your app and press the "Force Quit" button 
  • the Windows way: press "cmd + alt + esc" keys altogether, and it should prompt the "Force Quit" men

And as the little green guy would say, "May the Force Quit with you".