X is a
network protocol (usually carried over
TCP/IP or
Unix-domain sockets).
X clients, usually applications, utilities, or some such, usually deal with
X by using
Xlib or any of a number
of
libraries that sit atop Xlib (
motif,
Xaw,
gtk, etc.). They connect
to an
X server, which interprets
X protocol requests, performing the requested
drawing operations on some sort of
bit-mapped device, and sending
keystrokes,
mouse clicks, and other
events to the
client as appropriate.
The X Window System reference implementation, originally developed at MIT, was owned by The Open Group when this writeup was first written. Now it belongs to the X.Org foundation. Softwarewise, it consists of Xlib, the X toolkit (Xt), a number of clients, and a number of servers specific to certain video cards (or other display mechanisms). The X Protocol specifications are also considered to be part of the X Window System. At one time, motif (which is not available under a free licence, though clones such as lesstif exist) was considered almost a de facto part of X; once XFree86 (later XOrg) acheived massive popularity, however, this was not so much the case. Many proprietary Unix vendors that ship with X still include motif, however.
One of X's greatest assets is its network-transparence. Of course, it's really only transparent over a medium-speed connection or better; X over a modem is very sluggish. Most people who do not have philosophical objections to the entire X model feel that the colour model and font handling are its weakest points.
Those who do feel that the entire design of X is fundamentally broken usually point to NeWS; 8 1/2, the Plan 9 windowing system; Berlin; and whatever Apple's latest offering might be. They may be right. Quite possibly the only reason X triumphed over NeWS was that X was much more open.