[Taqwus]

XFree86 provides an implementation for the X Window System, which provides a nifty standard for networkable windows. One normally uses one of a variety of window managers for it which generally provide a consistent means for manipulating windows (minimizing them, resizing, whatever) plus usually right-click menus. mwm, twm, vtwm, tvtwm, fvwm, fvwm95, olvwm (?) etc.

KDE / Gnome go further and try to provide an entire desktop environment with stuff like a taskbar, some basic applications, perhaps a control panel for configuration, etc. Much easier for the novice to use. Plus, they provide widget libraries and APIs for easier and more consistent application GUI development.