Common Formats
A filename extension is an extra set
of (usually) alphanumeric characters that is appended to the end
of a filename to allow computer users (as well as various pieces
of software on the computer system) to quickly determine the
type of data stored in the file. It is one of several popular
methods for distinguishing between file formats.
File managers such as Windows Explorer can have applications assigned for almost every file name extension. For example, a text editor for .txt, a word processor for .doc or .odt, a web browser for .htm or .html, PDF viewer or editor for .pdf, a graphics program for .png, .gif or .jpg, a spreadsheet program for .xls or .ods, etc.
Under Microsoft's operating systems DOS and Windows, some extensions, including .exe, .com, .bat, and .cmd, indicate that a file is an executable. This is different from Unix operating sysems, where file name extensions are voluntary for executables, and instead permissions are used to decide whether a file is executable.
Filename extensions have been in use for decades, but they have gained common usage because the file systems included with DOS and Windows had severe limitations on filenames for many years, which strongly encouraged the use of filename extensions. Filename extensions can be considered as a type of metadata, though one of the most visible pieces of such information on modern computer systems.