xfree86-common contains the filesystem infrastructure required for further installation of the X Window System in any configuration.
Those wishing an X server only (with remote font services and clients) will also require the xserver-common package and an X server package (most likely xserver-xfree86).
The counterpart to the above configuration is a machine with the X libraries (the xlibs package), xbase-clients, a window manager, some X font packages, and likely many more client packages.
Those who desire a standalone X workstation (and/or are fuzzy on the concepts of X servers and X clients) will require both of the above sets of packages. For convenience, the "x-window-system" metapackage will include everything that is required for a standalone X workstation.
A number of terms are used to refer to the X Window System, including "X", "X Version 11", "X11", "X11R6", and "X11R6.4". The version of X used in Debian is derived from the version released by the XFree86 Project, Inc., and is thus often also referred to as "XFree86". All of the preceding quoted terms are functionally interchangeable in a Debian system.
Still confused? Install this package and then read the files in /usr/share/doc/xfree86-common/ for assistance.