A small program that takes over the display screen if there are no keystrokes or mouse movements for a specified duration. Screen savers were originally developed to prevent ghosting, the permanent etching of a pattern on a display screen. For older monochrome monitors, ghosting often occurred if the same pattern was displayed on a display screen for a long period of time. Screen savers would prevent this by either blanking out the screen entirely or by displaying a constantly moving image.
Modern display screens do not suffer so much from this problem. Today, therefore, screen savers are mostly an adornment, a way to liven up the computer. Many screen savers provide another benefit, hiding a user's work from would-be snoopers. These screen savers fill the display with an image or animation until the user enters a password.