I’m sure you must have used the Force Quit command at one time or another with an application that is not responding.
The keyboard shortcut to display the Force Quit dialogue is Command + ALT + ESC which brings up a list of open applications you can choose to Force Quit.
If the application that has become unresponsive is the application in the foreground, then a faster way to quit it would be to hold down the ALT + SHIFT keys while clicking on the Apple Menu () menu and select Force Quit.
Much quicker!