Installing XQuartz on Mountain Lion

What is XQuartz?

Mac OS X uses what is called X11 to run software through the X Window System. It has been around since OS X Leopard. It has a long history of open source support and has been running under the project header, XQuartz. Apple pretty much based their X11 app on this project. When Mountain Lion (OS X 10.8) came along, Apple no longer shipped X11 along with it.