There is a large amount of detailed information available about the X server, the clients that connect to it, and the assorted desktop environments and window managers.
/usr/share/X11/doc/ — contains detailed documentation on the X Window System architecture, as well as how to get additional information about the Xorg project as a new user.
man xorg.conf — Contains information about the xorg.conf configuration files, including the meaning and syntax for the different sections within the files.
man Xorg — Describes the Xorg display server.
http://www.X.org/ — Home page of the X.Org Foundation, which produces the X11R7.1 release of the X Window System. The X11R7.1 release is bundled with Red Hat Enterprise Linux to control the necessary hardware and provide a GUI environment.
http://dri.sourceforge.net/ — Home page of the DRI (Direct Rendering Infrastructure) project. The DRI is the core hardware 3D acceleration component of X.
http://www.gnome.org/ — Home of the GNOME project.
http://www.kde.org/ — Home of the KDE desktop environment.