Q: What is a file extension?
A: A file extension, also called a filename extension, is the suffix at the end of a filename, which indicates what kind of file it is. For example, you can tell that the file displayed on the left is a rich text file because of the the ".rtf" extension. Similarly, "readme.txt" is a plain text document, "computer.jpg" is a JPEG image file, and "document.docx" is a Microsoft Word document. It is helpful to learn the most common file extensions, since you will be able to identify common file types.
While most file extensions consist of three characters, some have fewer (e.g., .H, .DB, .PS) while others have more (e.g., .HTML, .TORRENT, .TAX2012). When you double-click a file, your computer references the file extension to determine what program should open it. If you change a file's extension, it may alter what program your computer uses to open the file. Therefore, you should be careful when changing file extensions. If you assign an invalid file extension to a certain file, your computer may not be able to open it. For example, if you change a ".txt" file to a ".jpg" file, it will open in an image viewing application rather than a text editor when you double-click it. The image viewer will most likely produce an error saying it does not recognize the file.
Since there are tens of thousands of software programs available, there are also tens of thousands of file extensions. While it is helpful to know the most commonly used file extensions, it is simply not possible to learn them all. Therefore, whenever you come across a file extension you don't recognize, visit FileInfo.com to find out what kind of file it is.
Updated: February 8, 2013