The SETI@Home screensaver is a popular screensaver that actually does something useful.

For most people, a screensaver pops up whenever you leave your computer unattended for more than a couple of minutes. It may simply be a blank screen, a specialized program such as SETI@Home, or it could be something as outrageous as dancing macaroni.

But what is a screensaver exactly? What purpose does it serve? How does it know when to start? In this article, we will look behind the screen and check out just what's going on.