From Mac Guides
Sometimes applications in OS X will hang or freeze. You will usually know this when you see the spinning pinwheel cursor (sometimes referred to as the beachball) for several minutes or more. In almost all cases, this will not affect other parts of the system, though it could slow the system down while waiting for the application to become unstuck. In a situation where an application is hung for good, or you don't want to wait for it to become responsive again, you can Force Quit it. Force Quitting an application will cause any unsaved changes in that application to be lost. The rest of the system, however, should continue to run uninterrupted. Some applications and processes, like the Finder, should automatically relaunch after being Force Quit. If the Classic Environment is being Force-Quit, then the entire environment will go away, with all applications and all unsaved data in those applications. This is partly due to the design of the older Macintosh OSs.
- Select Force Quit... from the Apple menu. Sometimes you will have to click to another application to get the Apple menu to respond. This will bring up the Force Quit Applications window; stuck applications may appear in this list in red. You can then select the misbehaving application and click the Force Quit button. Occasionally you will have to dismiss this window and repeat the process to really get the app to quit, but this should be rare. If necessary, you can Force Quit multiple applications from this window.
- Use the keystroke apple-option-esc to bring up the same Force Quit window.
- Control-click/right-click on an application's icon in the Dock. Usually you can simply quit an application from this menu. A stuck application will often say Force Quit in place of Quit, though you can hold down the option key if it does not.
The complicated ways:
- Activity Monitor will allow you to quit/force quit individual processes. Sometimes this will require an admin password. It should be used with caution, as Force Quitting a parent process will Force Quit all of its children processes too. For instance, Force Quitting the loginwindow process will end all user processes and dump the user back out to the Login Window.
- Using the Terminal, you can use the kill command along with a process's PID to kill that process. This is basically the same thing as using Activity Monitor above.