The extension tells what type file this is and what program it is sometimes associated with. Files with extensions of jpg, tif, png are picture files. However they are different types of pictures and if you changed a picture to another extension the computer would look at the file and say it is not that type when you try to open it. Common extensions of video files are mp4, fla, avi, wav and others. Audio files may be avi or mp3. Word documents have extensions of doc or docx depending on version of Word. Excel spreadsheet files have xls and xlsx. Text type files (files that have plain text in them, no font specifications, pictures, etc) are txt extension. Files that display web pages are html or htm. Usually extensions are three characters but sometimes four. In DOS days (the 1980s) all extensions were three characters but UNIX has allowed longer always and Windows does also.
Usually by default Windows does not display extensions but often says it is a certain type file, but that information comes from the extension. To display extensions in Windows open your main drive (usually C drive) in Computer (My Computer in XP) option in the Start menu. Then choose Organize, then Folder and Search Options, then the View tab and uncheck Hide extensions for known file types. By checking that box you can hide most extensions again.
Send your questions to Dwight Watt at email@example.com. He teaches at a technical college in northwest Georgia and does consulting work for businesses and individuals. His website is dwightwatt.com.