I think this must be one of the most misinterpreted jargon terms. As to which is which? I am talking about the desktop and the screensaver. People do seem to get mixed up, and hopefully we can put an end to the confusion.
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.
The screensaver is the process that happens when you have left the computer idle for a while; this is normally after about 10 minutes. When the screensaver is active, it takes up the whole screen and is normally something animated, or is a collection of photos being shown like a slideshow. When you press a key or touch the mouse, the screensaver will stop running and will return to where you were previously. See below for (poorly, but quickly put together) animated example.
I hope this makes a better understanding of which is which.
Here ends the Rant!