Q: What is a file extension?
A: A file extension, or 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 (e.g., .PDF), some have fewer (.H, .DB, .PS) and others have more (.HTML, .TORRENT, .TAX2016). When you double-click a file, your computer uses the file extension to determine what program should open it. The connection between a file extension and the default program that opens it is called a file association. You can change file associations in both Windows and macOS.
If you manually 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.
Because such a large number of software programs exist, there are tens of thousands of file extensions. While it is helpful to know the most commonly used file extensions, it is not practical to try and remember 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 and what programs can open the file.
Updated: December 7, 2016