In this edition of How Stuff Works , we will look behind the screen and check out just what's going on. A screensaver is really just an executable file , with the extension changed from .exe to .scr. An executable file is a file that the computer's operating system considers a program or application.
File extensions tell the computer what kind of file it is dealing with.
For example, winword.exe is a word processing application that can be loaded by the computer, while article.doc is a text file that can be loaded into the Microsoft Word word processing application. So we know that, in essence, a screensaver is a program. 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 as such, Windows makes it available as an option in the Display properties window. The screensaver file can be programmed in several different ways.