For most people a screensaver pops up whenever you leave your computer unattended It may simply be a blank screen or any picture or animation.But exactly it is executable file, with the extension changed from .exe to .scr so a screensaver is a program which can be execute by itself.
Screensavers were originally designed to protect monitors from phosphor burn-in. Early CRT monitors, particularly monochrome ones, had problems with the same image being displayed for a long time. The phosphors, used to make the pixels in the display, would glow at a constant rate for such a long period of time that they would actually discolor the glass surface of the CRT. This discoloration would then be visible as a faint image overlaying whatever else was displayed on the monitor. Advances in display technology and the advent of energy-saver monitors have virtually eliminated the need for screensavers.
By putting a screensaver file into the Windows or System directory and giving it a .scr extension, Windows knows that it should treat this file as a screensaver and makes it available as an option in the Display properties window.The screensavers that have a custom interface, and do not use the Display properties window at all, are less common. Usually, these screensavers do not have the .scr extension. They require that you install them using a setup program in order to configure them properly.
Programs like Screen Saver Builder allow you to combine images, animations and sounds to create your own screensaverIf you are a programmer, there are numerous online resources that supply you with technical information, and even source code, so that you can write your screensavers.