What is the Apple Menu used for?
The Apple Menu is a key location from where you can find out information about your Mac, run Software Update, enter System Preferences, kick off system life cycle events such as Sleep, Restart, Shutdown, and Log out of your session.

You'll see the Apple Menu no matter which application you're running because the Apple Menu never moves or goes away.  It is always located in the upper left corner of the screen.  The following examples show that the location of the Apple Menu remains despite using different application:

• About this Mac - Lists information about the installed OS version, memory and processor
• Software Update - Brings up the OS X software update application to check if any updates are available for your Mac
• System Preferences - Takes you to a special system configuration and settings window
• Dock - allows you to customize the Dock
• Recent Items - Lists the applications and documents that were recently opened or used.  The number of items listed can be configured in System Preferences.  Launch System Preferences, click Appearance, and then adjust the area called Number of Recent Items as needed.
• Force Quit - Bring up a window to allow you to forcefully quit an application (ideally used for unresponsive applications)
  
• Sleep, Restart, Shutdown - Self explanatory
• Log Out - Shuts down all running programs and documents (you'll be asked to save any unsaved items) and returns you to the Mac OS X login screen.

• March 15, 2009 - content revision, image updates