The X Window System is the graphical windowing system used by most Unix systems to provide users an easy to use enviroment beyond the basic command line interface. Red Hat Inc. uses the version crafted by the XFree86 Project. When reading documentation about the X Window Systems, you may see it documented as X, X11, X11R6, or XFree86. These terms are mostly interchangeable.
The XFree86 Project releases new versions of XFree86 about every six months, so depending on which version of Red Hat you have, you will have a certain version that may have been installed during the install (if you didnt chose not to have X).
As of this writing, the current version of XFree86 is 3.3.3 which was released around mid November 1998. Red Hat released RPM updates to it in December which I will document how to upgrade to.
The steps for upgrading XFree86 onto a Red Hat Linux system can be boiled down to the following steps.
XF86Setup