A string appended to a filename, consisting of a period followed by three alphanumeric characters. File extensions usually identify the application that can open or run them. For example, text files end with the extension .txt and are opened with Microsoft Notepad.
Other common file extensions include the following:
Associations between different file extensions and the programs used to open them are stored in the registry. Sometimes you might need to modify or remove a registered file extension. For example, if two different applications save files using the same file extension, you can easily modify file extensions in Microsoft Windows 95, Windows 98, or Windows NT by using Windows Explorer. Just select Options from the View menu to open the Options dialog box, and select the File Types dialog box. Create, remove, or edit file extensions as desired. Be aware that using this tool might negatively affect the ability of applications on your system to function, so modify extensions with care. In Windows 2000, you can do the same by selecting Folder Options from the Tools menu.
Graphic F-9. File extension.