||Wikipedia Definition - Original URL: http://en.wikipedia.org/wiki/Screensaver|
Before the proliferation of LCD screens, most computer screens depended on cathode ray tubes (CRTs). Images on a CRT monitor are generated using electron beams which are emitted from electron guns at the back of the tube, and manipulated by electromagnetic fields to form images line-by-line on the phosphorescent screen many times per second. In some situations the images displayed on the screen constantly change, but in other cases some areas of the screen, or the screen as a whole, change very little (the taskbar in MicrosoftWindows, for example). When the same image is displayed on a CRT screen for long periods of time, the properties of the exposed areas of phosphor coating on the inside of the screen gradually and permanently change, eventually leading to a darkened shadow or "ghost" image on the screen. Televisions, oscilloscopes and other devices that use CRTs are all susceptible to phosphor burn-in, as are plasma displays to some extent.
Screensaver programs were originally designed to help avoid these effects by automatically changing the images on the screen when the computer was not in use (thus "saving" the screen from burn-in). They can be usually set up to launch automatically, waiting a specified amount of time after the last keystroke or the last mouse movement made by a user. The screensaver then blanks the screen, or more commonly produces animation effects, thus avoiding any "fixed" images. The screensaver remains active until a user presses a key or makes a mouse movement. At that moment, the screensaver closes and the former screen contents are restored, allowing the user to work again.
For CRTs used in public embedded applications such as ATMs and railway ticketing machines, the risk of burn-in is especially high because a stand-by display is shown whenever the machine is not in use. Older machines designed without burn-in problems taken into consideration often display evidence of screen damage, with images or text such as "Please insert your card" (in the case of ATMs) visible even when the display changes while the machine is in use. Blanking the screen is out of the question as the machine would appear to be out of service. In these applications, burn-in is prevented by shifting the position of the display contents every few seconds, or by having a number of different images that are changed regularly.
Modern CRTs are much less susceptible to burn-in than older models due to improvements in phosphor coatings, and because modern computer images are generally lower contrast than the stark green- or white-on-black text and graphics of earlier machines. LCD computer monitors, including the display panels used in laptop computers, are not susceptible to burn-in because the image is not directly produced by phosphors (although they can suffer from a less extreme and usually non-permanent form of image persistence). For these reasons, screensavers today are primarily decorative or for entertainment, and usually feature moving images or patterns and sometimes sound effects.
One increasingly popular application is for screensavers to activate a useful background task, such as a or a distributed computing application. This is convenient because these applications only use resources when the computer would be otherwise idle.