The few characters appended to a filename that tell a user or application what type of file it is.

The extension can be determined by examining the filename and locating the last occurrence of the period or dot. This is typically the fourth character from the end of the name, and the following characters describe what kind of file it is.

Files ending with .txt, .Txt, .TXT and other variations are all files with Content-Type: text/plain. These ascii text files are readable by any text editor or viewer, such as Windows Notepad or *nix's vi. With *nix, most text files do not have extensions, though.

Some files do not have three character extensions, such as the Joint Photographic Experts Group .jpeg format. These must be shortened to .jpg when saved to a system that runs Windows 3.x or DOS.

Some files have more than one dot, as well. A file called does not have a ".windows.txt" extension, because only the characters after the last dot determine the extension of a file. In this case, the extension is .txt.