A long time ago, monitors had a problem with something called burn-in. If you left your monitor on for most of the day for months on end showing the same screen, the stream of particles that painted the picture of your desktop on the monitor would degrade the phosphor screen, resulting in a ghostly image that never went away.
While the problem of burn-in is either eliminated or mostly gone (depending on who you talk to), screensavers are still around. Originally meant to save your monitor from burn-in, they can keep you entertained or put pretty pictures up on your screen while you are away from your desk.
After Dark Classic, by Berkeley and Sierra, contains 22 screensaver modules designed to prevent phosphor burn-in on your monitor. Or just to keep you entertained.