In Windows 7, in the extreme right side of the taskbar, you will see a vertical rectangular portion.
Moving your cursor to this area will make the open windows transparent and let you preview your desktop. Clicking on it will show the desktop. This is called as the Desktop Preview feature in Windows 7.
But if you dont like this feature or hardly ever use it, you can always disable it.
To disable the Windows Preview feature in Windows 7, right click on the taskbar and click open Properties.
Under the Taskbar tab, you will see a checkbox to Use Desktop Preview. Un-checking it will disable this feature. Click Apply > OK.
Incidentally, right clicking on the Desktop Preview button will also present you with this option.