From PROBOTIX :: wiki
A post processor is a language definition file used to modify generic g-code such that it can be used on a particular cnc machine. They are just text files and can be opened and editied in notepad. They define inch vs metric, the file extension that the g-code is saved as, whether or not line numbering is used, the pre-amble and post-amble, and what each of different classes (rapid, feed, arc, etc) of moves looks like. Mach3, LinuxCNC, and the CNC USB Control Software can all use the generic g-code post-processors available with every CAM program.
The follow zip files contain the ArtCAM post processor for use with LinuxCNC or PlanetCNC software. A post is provided for Inch and MM. After you unzip the file, the folder will contain two post processors for ArtCAM. You will place these in the PostP folder under ArtCAM2013. When you restart ArtCAM the added posts will be available as a machine language option.