With the Force Quit command in OS X, you can escape from “frozen” applications on your Mac without shutting down the whole computer or disrupting work in other programs.
If an application hasn’t responded for a while to mouse clicks, trackpad scrolling, or other persuasive actions, click on the Apple menu and select Force Quit.
A pop-up window lists all the applications you currently have open. Stalled applications are listed in red and say (not responding) after the application name. Scroll up and down through the list with the arrow keys, or just click on an application name to select — then click Force Quit to exit only that program. You can restart the application from your Dock or Applications menu, but any unsaved changes may be lost.
But there’s an even easier Force Quit trick: Simultaneously press Command-Option-Esc and the Force Quit pop-up appears — an especially useful trick if your Finder has frozen and you can’t select the pull-down Apple menu.