Wikipedia
XQuartz (formerly and often still informally referred to as X11.app) is Apple Inc.'s version of the X server, a component of the X Window System, for Mac OS X. The X Window System is a standard framework for creating applications with a graphical user interface on Unix-like operating systems. XQuartz is therefore a required component that allows cross-platform applications, many of which were not specifically designed for OS X, to run on it. This includes numerous scientific and academic software projects. The name derives from Quartz, the OS X graphics framework, to which XQuartz connects these applications.
The current version of XQuartz is a DDX (Device Dependent X) included in the X.Org Server and implements support for hardware-accelerated 2D graphics (in versions prior to 2.1), hardware OpenGL acceleration and integration with Aqua, the Mac OS X graphical user interface (GUI).
X11.app was initially available as a downloadable public beta for Mac OS X v10.2 and later included as a standard package for Mac OS X v10.3, which can be downloaded from Apple's website. In Mac OS X v10.4, X11.app was an optional install included on the install DVD. Mac OS X v10.5, Mac OS X v10.6 and Mac OS X v10.7 installed X11.app by default, but from OS X Mountain Lion (10.8), Apple dropped dedicated support for X11.app, with users directed to the open source XQuartz project (to which it contributes) instead.
In Mac OS X v10.4, Apple's X11 implemented X11 protocol release 6.6 (X11R6.6). This implementation includes an XFree86 4.4 based X11 window server, Quartz rootless window manager, libraries, and basic utilities such as xterm. "Rootless" means that X window applications show up on the Quartz desktop, appearing like any other windowed Quartz application (that is, not in a virtual desktop contained within another window). In Mac OS X v10.5, X11 was updated to use X.Org Server (X11R7.2) rather than XFree86.
The source code for X11 is available from Apple. Some source code is available under the Apple Public Source License while the bulk is licensed under the MIT License.
As of version 2.7.9, XQuartz does not provide support for high-resolution Retina displays to X11 apps, which run in pixel-doubled mode on high-resolution displays.