A pattern or picture used to represent the desktop surface (screen background) in a graphical user interface (GUI). Although many wallpaper options come with each operating system, such as random images from the user's folder, any single image can be used. Many cellphones allow custom wallpapers for their screen backgrounds.
Live wallpaper displays 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. With Windows XP, Microsoft decided to end the mystery and just call it "Background." See desktop