A screen saver is a moving picture or pattern which is set to automatically appear on your screen if you don't use the computer for a certain period of time.
When CRT monitors were used, if you left your monitor switched on but didn't use the computer for a long period of time, whatever was on the screen at the time could be burned into the phosphor of the screen. For the rest of the life of the monitor, you would see a ghost of that image on the screen.
For this reason, screen savers were invented.
With modern TFT monitors, screen burning isn't really a problem. However, screen savers remain popular as people like to be able to customise their computer.
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Click on this link: Screensaver