A pattern or picture used to represent the desktop surface in a graphical user interface (GUI). Also called the "background," many wallpaper options come with each operating system, such as a single image or random images from one of the user's folders. Smartphones also allow a variety of backgrounds to be used on the home screens.
Live wallpaper renders some moving object or continuous scene change for people who like the glitz of animation. However, in a portable device, live wallpaper is a battery drain.
Why Cover a Desktop with Wallpaper?
Years ago, someone who had too many sleepless nights programming must have thought it a great analogy because a screen desktop is vertical like a wall. Starting with Windows XP, Microsoft decided to end the mystery and just call it "Background." See desktop
and screen saver