If you came from a PC upbringing like me, you have probably been Ctrl+Alt+Del brainwashed when it comes to frozen screens and unresponsive programs. Now that I've graduated into the Mac world of computing, I've had to train myself on all of the Apple key commands.
Rather than pressing Ctrl+Alt+Delete to terminate unresponsive applications, on a Mac you press Command-Option-Escape (the pic to the left has an arrow on the Apple command key). Once you press these keys, select any application from the Force Quit window and click Force Quit. Or you can right click on the application in your Dock and then select Force Quit too.
The Apple/command key can also be used to copy (Cmd+C), paste (Cmd+ V), cut (Cmd+X), and undo (Cmd+Z).
For more tips on how to accomplish the same tasks on a Mac as a PC click Switch 101.