It's called 'show desktop' - it tells you that if you hover your cursor over the tiny vertical bar at the right end of the taskbar.
areo peek
Taskbar.
True
Desktop
The desktop is the main computer screen that is visible when there are no open windows. It usually has a color, graphic, or photo background with icons for various shortcuts or documents. It is also associated with the taskbar (Windows) and dock (Mac).
[a]Quick launch [b]Systray [c]Taskbar [d]Time
It varies depending on your operating system, but in general the bar along the bottom is the taskbar, the picture you see is called the desktop background, and the programs on the desktop background are called icons.
taskbar
The portion of the screen that displays when windows XP is loaded is called the Desktop. It features icons for different programs, the Taskbar and the Start Button.
You right-click on your taskbar and then click on properties. Then you un-select 'Keep the taskbar on top of other windows'.
The main feature of Windows Vista Aero is the way Alt-Tab changed. Instead of small icons it now displays the available tabs in a 3D environment. Another associated feature is the windows popping up when hovering over the tabs in the taskbar.
Actually the wallpaper is just the background screen of the desktop and the desktop is where you can see the taskbar, icons and the wallpaper. Wallpaper is only the background while the desktop is the whole screen including the other things in it (ex: icons, files, windows etc.)
There are some differences. A new Mac Dock-like taskbar is bigger and more touch-sensitive. There also is Peek, (hold your mouse over a taskbar button and see it as if it were open, or move your mouse all the way to the right of the taskbar to "peek" at the desktop), Snap (drag your windows to different sides of the screen to automatically "snap" them to the sides), and Shake ("shake" the window and all other windows hide, too).