Filename extensions are a form of identifier for programs and files; a text file might have .txt or .doc after its proper name, denoting either a plaintext file or a MSWord document with higher-level formatting. The extension allows an operating system to classify and sort files according to their function; the OS also associates other programs with the files based on the extension. For example, on a Windows PC the file example.txt would associate with Notepad, a simple text editor, while example.doc would associate with MSWord or similar, a more powerful editor. Example.rtf might associate with several programs because the .rtf denotes Rich Text Format, which is more universal. By identifying the file according to a generic list, the OS can store data about the file as well as remember it for searching. Some filename extensions have more than one function; a .exe file can open as a runtime application, link as a shortcut to a different application, or operate as a batch file to run one or more separate programs (a batch file is often .bat, but sometimes is listed as .exe to avoid confusion).
Filename Extension indicates the type of file and in some operation systems it is used to order the system to do some specific function for instance extensions such as EXE, COM or BAT indicate that a file is a program executable.