A quick way to switch between open applications is by using the Command + Tab keyboard shortcut. This will display a transparent bar of icons in the middle of your screen.
You can keep pressing the shortcut combination to cycle through the open applications on your computer while the bar is displayed on your screen.
The Command + Tab keyboard shortcut can be used to quickly quit open applications. Just press the Q key on your keyboard while an application is highlighted in the transparent icon bar that comes up after you use the keyboard shortcut.
See It In Action
Application Switcher (31s, 7.5 MB)
This movie shows you how to use the application switcher, including how to use it to close an application you have open.