For a while now, Windows users have had a Show Desktop button on their Taskbar. In Windows Vista and earlier, it was a shortcut on the Quick Launch toolbar. On Windows 7, it is an actual part of the Taskbar.
Under Mac OS X, there are some shortcuts to show the desktop.
Hitting F11 will move windows off the Desktop, but as soon as you open anything - everything shoots right back onto the Desktop.
Holding the Option and Command keys when clicking on the Finder icon in the Dock will hide all windows - except for Finder windows.
Someone has made a program that when clicked, will hide all open windows and minimize all Finder windows, giving you a truly clean Desktop to work with.
The "Show Desktop" program can be set to show up on the Dock (like where you would find it on Windows), or it can run from the Menubar at the top of the screen.
You can download Show Desktop for Mac here:
http://www.everydaysoftware.net/showdesktop/