Although the X in "Mac OS X" is not the same X as in "The X Window System," you can get them to play nice together.
Most Unix systems use the X Window System as their default GUI. (We'll refer to the X Window System as X11 instead of X, to avoid confusion with Mac OS X.) X11 includes development tools and libraries for creating graphical applications for Unix-based systems. Mac OS X does not use X11 as its GUI, relying instead on Quartz (and, on compatible hardware, Quartz Extreme), a completely different graphics system. However, Apple's own implementation of X11 for Mac OS X, based on the open source XFree86 Project's X11 (http://www.xfree86.org/), was initially released as a beta for Jaguar and is now bundled with Mac OS X Panther as an optional installation. Apple also provides an X11 software development kit (the X11 SDK) on the Xcode Tools CD that ships with Panther.
This chapter highlights some of the key features of Apple's X11 distribution and explains how to install Apple's X11 and the X11 SDK. It also explains how to use X11 in both rootless and full-screen modes (using the GNOME and KDE desktops). You'll also learn how to connect to other X Window systems using Virtual Network Computer (VNC), as well as how to remotely control the Mac OS X Aqua desktop from other X11 systems.
From Aqua to X11, there's no shortage of graphical environments for Mac OS X. The operating system's solid Unix underpinnings and powerful graphics subsystem make it possible for developers to support alternative graphical environments. For this reason, a humble iBook can make a fine cockpit for a network of heterogeneous machines!