The term desktop wallpaper (as the name implies) refers to an image used as a background on your computer screen. The term wallpaper is the term used by Microsoft, while Macintosh avoids using this term not to mix the metaphors, and instead uses the term "desktop picture." In the public eye, "desktop wallpaper" terminology, or just "wallpaper" is mostly used to indicate a computer screen background image.
The desktop wallpapers are used to fill in the otherwise blank and monotonous computer screens with an image of your desire. Some people choose a picture of their favorite celebrity, car, or a vacation-desired destination. Others choose animated wallpapers, 3D wallpapers, nature scenes, computer generated images of some fantasy world, favorite movie, or a white sand beach with breathtaking ocean views. The desktop wallpaper is purely one's desire.