"Show Desktop" is a shortcut which is present in Quick Launch toolbar in Windows taskbar. Once you click on its icon, it minimizes all running programs to taskbar so that you can view the Desktop.
Since Microsoft integrated the Quick Launch toolbar in taskbar in Windows 7, you'll no longer see the old "Show Desktop" icon. But that doesn't mean it has been removed in Windows 7.
Actually the location and look of "Show Desktop" has changed in Windows 7. Now its located at the end of taskbar in form of a small rectangle.
It does 2 jobs now:
1. When you move your mouse cursor over it, it hides all running programs windows and shows only windows outlines so that you can peek behind all open windows . Its called "Aero Peek" feature.
2. When you click on it, it does the same job as the old shortcut used to do. It minimizes all open windows and restores them if you click again.