On your computer, everything is stored as a file.
Every file has a file extension that tells the world what kind of a file it is.
A file is simply a packet of information such as a picture or a document. If you want to organize your files and put them into nice neat little groups, you'd use some folders . Folders don't come with file extensions because because they're not files. Make sure the Hide MS-DOS File Extensions box is not checked. They are the backbone of Web Pages.