A screensaver (or screen saver) is software that controls a monitor of the host computer with the intent of preventing screen burn-in for a screen susceptible to it. Generally, a screensaver starts controlling a monitor when the computer has been idle for a designated period of time and fills the screen either with black (all pixels off) or with changing graphics that tend to prevent each pixel from being on for a long time. Although monitors were commonly constructed with screen technology that was susceptible to burn-in (CRT and plasma), most modern monitors are LCD which are not. Another modern technology, OLED, is susceptible.

In addition to the feature described by its name (saving a screen from burn-in), a screensaver may provide other features. It may provide physical security by requiring a password to exit the screen control mode. Some use otherwise-idle computer resources to do useful work, such as processing for volunteer computing projects.

Many modern devices such as televisions and other digital entertainment devices include a screensaver.

A monitor controlled by a screensaver consumes the same amount of power as when the screensaver is not controlling it, which can be anywhere from a few watts for small LCD monitors to several hundred for large plasma displays. Most modern computers can be set to switch the monitor into a lower power mode, blanking the screen altogether. A power-saving mode for monitors is usually part of the power management options supported in modern operating systems, though it must also be supported by the computer hardware and monitor itself.

Using a screensaver with a flat panel or TFT LCD screen not powering down the screen can actually decrease the lifetime of the display, since the fluorescent backlight remains lit and ages faster than it would if the screen is turned off and on frequently. As fluorescent tubes age they grow progressively dimmer, and they can be expensive or difficult to replace. A typical LCD screen loses about 50% of its brightness during a normal product lifetime. In most cases, the tube is an integral part of the LCD and the entire assembly needs to be replaced. This is not true of LED backlit displays.

Thus the term "screen saver" is now something of a misnomer – the best way to save the screen and also save electricity consumed by screen would simply be to have the computer turn off the monitor. Screensavers displaying complex 3D graphics might even add to overall power draw.

Before the advent of LCD screens, most computer screens were based on cathode-ray tube (CRT) technology. When an image is displayed on a CRT screen for a long period, the properties of the exposed areas of the phosphor coating on the inside of the screen gradually and permanently change, eventually leading to a darkened shadow or "ghost" image on the screen, called screen burn-in. Televisions, oscilloscopes and other devices that use a CRT are susceptible to phosphor burn-in, as are plasma displays to some extent. Screensavers were originally designed to help avoid these effects by automatically changing the images on the screen during periods of user inactivity.

For CRTs used in public, such as an automated teller machine (ATM) and railway ticketing machine, 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 not a valid option as the machine can be perceived as out of service. In these applications, burn-in can be prevented by shifting the position of the display contents every few seconds, or by having a number of different images that are changed regularly.