What is the file format for Windows screensavers?
There is no unique file format for screensavers under the Microsoft Windows operating system. Rather, screen savers are simply executable files that have had their extension renamed from .EXE to .SCR so that Windows recognizes them as being screensavers. For example, when you open the Screen Saver Settings panel, Windows will start searching through several folders on your hard drive, looking for files that have the .SCR extension. Any such files are then added to the list of screensavers, and can subsequently be selected, previewed and configured.
Not all executable files can simply be renamed to act as a screensaver, however. When Windows finds a file it deems to be a screensaver, it will nonetheless expect it to respond appropriately to certain command line arguments, specific to screensavers. If it does not, the executable file will not be run as a screensaver.
The .SCR extension, then, is uniquely identified by Windows as belonging to a screensaver file, but the file format ultimately does not differ from other Windows applications that have the .EXE extension.