File extensions, also called file extension or file name extension, is a suffix added to file names to identify them in their file format. The file extension is a letter, a combination of letters, or a combination of letters and digits that has some relationship to the file format. The suffix added to the file name is separated from the file name by a period.
The file suffix may be one, two, three, or more letters, such as *.ad, which may represent a default for a Unix user file, or *.txt, which represents a text file. Similarly, letter- digit combinations such as *.mp3 or *.mp4 are associated with the MP3 file format or the MP4 file format. File extensions are not standardized. They can therefore be freely assigned. This results inevitably, since letter combinations with three letters have only a limited name space, even if this is over 10,000. This leads to the fact that identical file extensions are used for completely different file formats. The example of *.aas file extension shows that this file extension is used for Archos Signed/Encrypted Data, Audible Words File, Autodesk AnimationSetup and for other file formats.
In some operating programs file extensions are mandatory, as in the DOS operating system or Windows, and in others, as in Unix, they are optional. Some operating systems use the file extensions for other purposes.