File created by Mercurial, a control management developer tool; contains the differences between two different versions of the same text or source file; used by developers that are making changes to different versions of the same file; enables updating of older text files to newer version text files.

The "patch" command searches for the line that starts with the string diff -, once it finds that string, it knows that that's the beginning of the patch content. Mercurial is mostly written in the Python and C languages. NOTE: Also known as .DIFF files because they were originally created by the diff command.