The desktop is the place where you see all your usual desktop actions and folders. For example, your documents folder, computer, Internet Explorer, email, recycle bin, etc. It also shows the start button and taskbar (the thing that runs along the bottom of the screen).
The desktop also normally has a background of the users choosing, such as a personal photo of family or a place, or it can be the usual system kind, a specific colour or mountains, clouds, a lush green field, etc. See below of example of desktop and desktop background. For further info, read this posting from last year here.
