Wallpaper is a (usually still, but not always) picture you put in the background of your screen view. A theme is a whole load of settings, inc. wallpaper, that you can set up together. A screensaver, not as vital since monitors changed, but still useful to indicate the computer is on, is a continuously changing pattern that shows when you have left the computer on but aren't actively using it. Originally run to avoid any single part of the phosphorus screen showing brightly and burning off the phosphor, thus leaving a permanent unwanted image.