XQuartz (usually referred to as X11.app ) is Apple's implementation of the X Window System ( X11) for Mac OS X. Apple's implementation of X11 based on the X.Org server and extends this among others to the hardware accelerated 2D display Quartz, hardware-accelerated OpenGL and integrates it better in Aqua, the graphical user interface of Mac OS X.
X11.app was available for Mac OS X v10.2 in a beta version and has become a standard package for Mac OS X v10.3, which can be downloaded from Apple's website later.
In Mac OS X v10.4 X11.app was included as an optional installation package on the DVD. Here X11 Version 6.6 ( X11R6.6 ) was used as a base. This implementation included a based on XFree86 4.4 X server, a window manager, the " rootless ", and basic utilities such as xterm. " Rootless " means that the X applications appear on the quartz desktop, where they appear like any other application in a normal window and not him a dedicated environment such as in Virtual PC.
In Mac OS X v10.5 through Mac OS X v10.7 X11.app is installed by default. It has been rewritten to use the more popular X.Org server ( X11R7.2 ) instead of XFree86.
Since OS X v10.8 ( Lion Mountain ) is X11 is not part of the operating system more. Apple supports instead the development of XQuartz and forwards on this external project.