The quick Command+Option+Esc keyboard shortcut gives you access to a list of running apps and lets you force-quit the if necessary. You can force-quit several apps as well, or use this as a quick app switcher. There are also other ways to force-quit apps, though in normal use you should always try to quit an app normally.
Good video, nice and concise, you're a good teacher :)
Nice video. I did not know about selecting multiple applications in the Force Quit dialog box or the methods in the Dock.