The extension defines the file ‘type’, e.g. text file, executable file, database file or graphic file or …. Actually a remnant from old DOS 8.3 filename limits, it has proven its usefulness over the years and today even normally ‘extension-free’ systems like UNIX use more and more extensions. An extension lets not only the user easily find out what sort of file it is, but even the OS (or other programs) can easily associate certain file-extensions with certain programs, thus e.g. starting a certain application when the user clicks on a certain (data-)file. That’s the theory. In practice it shows that the combinations out of 3 letters are limited and thus there are ambiguities. Since there are absolutely no further investigations done by the OS what concerns the file-type, misunderstandings can happen. Indeed they happen more and more, the more identical extensions are used by different programs. Although today there is no more need to reduce the extension to 3 characters, it seems most programmers stick to it. So even inexperienced users should be aware of the fact that a .doc file – from a friend, out of the net – needs not necessarily be a Word document, that dll’s aren’t limited to the dll-extension (Microsoft themselves in their ultimate wisdom breaks this rule often), that some extensions are used by ten different programs (with differing content well understood!). So if you got a file with an unknown extension or with a well known extension that the associated program can’t understand, this extension list can be a first hint what sort of file it may be. If the here listed program too can’t open your file, further investigation is necessary. There are automatic file-recognition programs in the net, but their capabilities are limited. Most of them use only the extension to find out what the file actually is. Experienced users will use ‘dump’ or ‘list’ to find out what sort of program it is (for dump or list see the ‘misc’-page of www.icmasterdata.com . But a lot of knowledge and skill is necessary to find out in this way what the file actually is. Further information on file formats can be found on e.g. www.wotsit.org (very special information normally thought only for programmers To hide file extensions (the three-letter suffixes attached to all files) for some, but not all, file types, select Settings from the Start menu, Folder Options, then click the File Types tab. Select the file type you wish to modify (for example, Text Document), and click the Edit tab. Check the Always Show Extension box, then click OK twice. (Explanation see below. It should be noted that there are other extensions which can be dangerous too. These are considered Level 1 files) The two with an asterisk not official. On a pure Dos32/Windows(98 (95 SR2) and later) level it is possible to use other characters (eg above 80hex, eg german/swedish umlauts etc), but its not guaranteed that all programs can handle them. Explicitely forbidden are these characters: <, >, : ‘ / | FAT (=DOS/Windows) allows paths of up to 260 characters, including the terminating special character. Do remember, though, that a path includes the full filename, so if you happen to have a filename that is 255 characters long, you will only have 4 characters left for the path (the last char is reserved for terminating the string). Source.