Every Mac OS X user has probably seen his cursor change in an spinning beach ball of death. This normally appears when an application is freezing up after more than a few seconds (depending on what OS version you are running), and it normally doesn’t respond anymore. In most cases this isn’t really a problem, it will show a crash report with the possibility to restart the application again. It could however happen that an application doesn’t do a single thing, and than you may want to use the force close option.
When an application stops responding, you can force close it with three ways, it however may depend on the state your system is currently in.
The first option can be found in the Apple menu: ▸ Force Quit
The second option is to right click on the icon of your application that is frozen, and the stop button should be replaced by the force-quit option. If this isn’t the case, you can press the Option-ALT ⌥ button.
If none of these options work, try pressing the following combination Command ⌘ + Option-Alt ⌥ + Esc and select the application that you want to stop.
A fourth option could be using the ‘kill’ command in Terminal or Activity Monitor, these methods should only be used by advanced OS X users.
When an application stops responding, it normally shouldn’t have any effect on other applications as it runs in an special sandbox mode.