A utility that was originally created to prevent a CRT from being etched by an unchanging image. After a specified duration of time without keyboard or mouse input, it blanks the screen or displays moving objects. Pressing a key or moving the mouse restores the screen. Screen savers can be password enabled so that no one else can gain access to the computer when users leave them running.
More Now for Fun and Security
With the vast improvements made to CRTs over the years, it would actually take many hours to burn an image onto a late model CRT. In addition, LCD flat panel monitors do not use phosphors and are not subject to burn-in. However, the security aspect as well as the entertainment they offer (swimming fish, flying toasters, etc.) makes them a popular and worthwhile utility. See screen burn