Higher-level applications can draw graphics to the screen
and receive input from the keyboard and mouse by just talking to the X
Window System. Individual applications don't have to know anything
about how the hardware actually works. X is a standard component of
most Unix-derivatives nowadays.
X does does this in a network-transparent fashion, which means that
applications running under the X system can be running anywhere on the
network.
The X Window System does not specify a policy for either the user
interface used by each of the applications being displayed or by the
decoration frame and window managing features. X actually relays the
responsibility for managing windows to a special application called
the Window Manager
The Window Manager controls the placement and appearance of windows on
the screen. It works in coordination with X and instructs X where and
how to draw windows. There are many window managers available that
offer different customization options, but they all perform the same
basic functions.
This is the way most people use X these days: the basic windowing
system, a window manager, and some X applications.