A file name extension is a file name suffix consisting of limited alphanumeric characters which communicate a file’s format. These extensions help users identify a file’s format without having to use it, as well as find out whether they have the right software to use it. Examples of file name extensions are exe, txt, jpg, png, docx, etc.
In programming, the term extension refers to a file containing code that expands capabilities of programs. An example of these extensions are browser add-ons.