Screen savers and lock screens are common applications found in many operating systems. The original purpose of the screen saver was to save the screen from image burn-in. This was particularly an issue with older cathode ray tube (CRT) terminals which were susceptible to phosphor burn-in. An image that was on the screen for some (long) time would leave a ghost image on the screen permanently. A screen saver would instead display a regularly changing image. The popular xscreensaver provided an impressive range of weird and wonderful patterns to display on the screen and save it from burn-in, all the while using compute power to generate some of the intricate patterns.
As of about 2011 though, the technology used for screens has changed significantly. With the development of liquid crystal displays (LCDs) the issue of burn-in has considerably reduced (and some say been solved). There is no longer any need to use fancy screen savers. In fact using one will consume power whilst simply automatically turning off the screen is more environmentally friendly. The gnome-screensaver, for example, will simply blank the screen after a period of time and put it into power saver mode.
The extra functionality often found with a screen saver is to lock the screen to prohibit unauthorised access whilst allowing a user to remain logged on.
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