Screensavers

Screensaver software was originally designed to protect computers displays when the machines were not in use. When one image is displayed on a CRT screen for a long period of time, the phosphor coating on the inside of the screen changes, eventually leading to a "ghost" image. All CRTs are susceptible to phosphor burn-in (so are plasma displays to some extent).

     The first screensaver was written for the original IBM PC in 1983. It simply blanked the screen after a period of inactivity. After Dark was a very successful screensaver program that was first released in 1989. Of the animated modules included, two of the most popular were the fish bowl and the flying toasters.

     Today, CRTs are much less susceptible to burn-in. LCD screens, including laptop displays, are not susceptible to burn-in (although they can suffer from temporary image persistence). Screensavers today are used primarily for entertainment, privacy, and security.

     There is also downside to screensavers. They keep the display on and using power, as opposed to taking advantage of energy-saving options that can cause a monitor to turn off after a period of inactivity. With an LCD screen, using a screensaver instead of powering down can reduce the lifetime of the display, since the backlight remains lit and ages faster.

(Paraphrased from Wikipedia)