Aero Peek is a feature in Windows 7, which allows you to take a quick peek on your Windows desktop without actually switching to desktop. To use Aero Peek, move your mouse cursor on the rectangular area on the extrem bottom-right as shown.
If just click on this rectangular area, then you would switch to Windows Desktop. This is equivalent to the Show Desktop icon in Quick Launch area of some older versions of Windows (XP and older).
To turn off the Aero Peek feature, you can right-click on this rectangular area and un-check Peek at desktop as shown :
If later you want to turn on Aero Peek back, you can right-click on this rectangular area and check Peek at desktop.