A screen saver is an animated image that is activated on a personal computer display when no user activity has been sensed for a certain time. The original purpose of a screen saver was to prevent burn-in (the burning of an image into the phosphor inside the cathode ray tube after hours of the same image being rescanned). In fact, today's CRT display technology makes burn-in unlikely except under extreme conditions. (On larger displays used for room presentations, burn-in is still a possibility.)
Windows comes with built-in screen savers that you can select. If you purchase your computer with the operating system already installed, a screen saver may have been selected for you. You can set the screen saver using the task bar by selecting Start, then Settings, then Control Panel, then Display, and then Screen Saver, which lets you select one of the provided screen savers or one you have added to the Windows screen saver directory. The Mac also lets you set up a screen saver.