Windows uses a file's extension--the part of the file name after the period--to identify what program should open it. When you double-click, say, a .docx file, Windows checks to see what application is associated with that extension (probably a word processor) and opens the file in that program.
You can change these associations yourself, and you can associate multiple programs with a single extension. In fact, there's a good chance Windows has already done that for you.
You can change these associations yourself, and you can associate multiple programs with a single extension. In fact, there's a good chance Windows has already done that for you.