Screensaver

A computer screen is usually a CRT (Cathode Ray Tube). This means that images are generated using electron beams which are "launched" from the back of the monitor and "draw" images continuously on the screen.

Most computer programs paint images in the screen. Some of these images (letters, pictures, animations, menus) are usually moving or changing, and never stay in the same place for long. But some portions of the screen (like the Start bar in Microsoft Windows, or the typical upper "score" bar of some videogames) are always in the same place, sometimes for hours or even days or months.

These sort of images, continuously drawn in the same place for a long time, could damage the screen because the electron rays are always hitting the same point of the screen. Damage would consist in poor image quality, and those "fixed" images could remain in the same place like "ghost lines" even if the image eventually changes. You could observe this effect in some old classic videogame machines; after some years showing always the same images, many of them feature those "ghost images".

Screensaver programs help avoid these effects by automatically changing the images on the screen when the computer is not in use. They can be usually set up to launch automatically, waiting a number of minutes after the last keystroke made by a user. Then the screensaver could switch the image to black, or maybe make some animation effects with images and colors, thus avoiding any "fixed" images. Some screensavers even feature sound effects.

The screensaver keeps working until a user does a keystroke or a mouse movement. In that moment the screensaver closes and the screen contents return to the previous ones, to allow the user to work again. Some screensavers can be programmed to ask users for a password, for privacy purposes ("lock the station").