from apples website
About X11 for Mac OS X
An implementation of the X Window System that makes it possible to run X11-based applications in Mac OS X. Based on the open source XFree86 project — the most common implementation of X11 — X11 for Mac OS X is compatible, fast, and fully integrated with Mac OS X. It includes the full X11R6.6 technology including an X11 window server, Quartz window manager, libraries, and basic utilities such as xterm.
Note 10.4 customers can install X11 by using the Tiger DVD installer disk.