A file extension is the suffix at the end of a filename that tells a computer, and the computer user, which program is needed to open the file. Also called a filename extension, this suffix preceded by at least one period, is generally one to five characters long but the norm is usually three characters in length.

Considering that there are thousands upon thousands of software programs available, and each one has a file extension if its own, it wouldn’t be possible to learn each one. You will, however, find yourself remembering extensions to more commonly used programs. It is a good idea to familiarize yourself with those used most frequently so that you can accurately recognize the files. Some of the ones you may come across more often are .DOC, which is usually for Microsoft Word documents, .TXT for text documents, .JPG for JPEG image files and .EXE for executable files, or files that run a program. If you come across a file extension that you don't recognize, a quick Internet search should be able to reveal it's associated software program.

It’s helpful to make sure your computer shows a file extension name for several reasons. Malicious programs can be sent with executable files with double extensions so that you are unaware of what type of file you’re really dealing with. Opening a nefarious file could expose your computer to a computer virus that could damage your computer and its data.

To configure your computer to display file extensions you can complete the following steps. For Windows XP users, open any folder such as My Documents, select Tools, Folder Options and then the View tab. Scroll down to where it says “Hide file extensions for known file types” and be sure this option is not selected. Finally choose “Apply to all folders” and close the window. To do the same in Windows 98, open My Computer, then choose View, Folder Options and select the View tab. Find the entry that says “Hide file extensions for known file types” and make sure it’s not selected. You will now be able to see the extensions in folders and in other programs from Microsoft such as Outlook.

While it is possible to change a file extension, doing so may cause the file to not open properly, or perhaps even at all. Some programs are able to read different types of extensions, but others may not know how to recognize the change. For this reason it’s advised to use caution when renaming a file extension. Make sure you do some research within the program you’re using before making any modifications to a file extension.