Screensavers are used when your computer is idle. You can run animations via a screensaver when you are not working. It exists right from the days of Windows 3.1. In this video, you will learn the steps required to use Screensaver in Windows 7.
During 1990-2003, CRT monitors were used to work with the PCs. These monitors produce lot of electrons which will ultimately damage the picture tune when the display is left idle for a very long time. Hence, in order to run something, Screensavers were invented by Microsoft.