In the DOS/Windows computer world, and UNIX as well, almost every file (anything on your computer that isn't a folder is a file in this context) must have some kind of extension. Example: index.htm would be a filename, where ".htm" is the file extension. On a PC in particular the operating system needs an extesion in order to determine what kind of file it is, and what to do with it when it is activated. With the internet, you may see extensions like .exe, .cgi, .asp, .htm, .jsp, .cfm, .tam, .php, .shtml, .pl, and many others. It is important to note that in some cases you have to be aware of the proper extension to use for a file depending on the environment in which the file will be used.