A file extension or suffix appears at the end of the file name and indicates what type of file it is. For example, a file with the extension ".txt" indicates the file is a text document. Another example is ".docx", which is a Microsoft Word extension.
The average file extension is three characters but can be as short as one character and longer than 20 characters. Some long file extensions are used to specify a particular program be used to open the file. For instance, ".TAX2011" is used to specify a file that opens in TurboTax 2011, and ".DESKTHEMEPACK" indicates a file to be used as Windows 8 desktop theme. The file extension is also used to determine which program icon to use for the file.
Windows and Mac OS allow the file extension to be changed manually, which can also change the default program that opens the file. Some files may not work in certain programs, so if the extension is changed, the file may not work properly or may not work at all. An example is changing a .txt file extension to .psd, which is a Photoshop extension — Photoshop does not recognize .txt files and cannot open them.