What is a File Extension?
A file extension, or filename extension, is a suffix at the end of a computer file. It comes after the period and is usually two-four characters long. If you’ve ever opened a document or viewed a picture, you’ve probably noticed these letters at the end of your file.
File extensions are used by the operating system to identify what apps are associated with what file types—in other words, what app opens when you double-click the file. For example, a file named “awesome_picture.jpg” has the “jpg” file extension. When you open that file in Windows, for example, the operating system looks for whatever app is associated with JPG files, opens that app, and loads the file.