The X Window System is a graphics system primarily used on Unix systems (and, less commonly, on VMS, MVS, and MS-Windows systems) that provides an inherently client/server oriented base for displaying windowed graphics. It provides a public protocol by which client programs can query and update information on X servers.
The representation of "client" and "server" appears backwards from nomenclature used in most client/server systems.
Usually, people expect the "local" part to be called a "client," and for the "server" to be a service hiding in the back room. This nicely represents the way database applications usually work, with many "clients" connecting to a central database "server."
X reverses these roles, which, as the locations of the hosts are reversed, is actually correct:
An X server is a service that manages a video system (and possibly other "interactive" I/O devices such as mice, keyboards, and some more unusual devices).
The X server thus typically runs on a user's desktop, typically an otherwise non-powerful host that would commonly be termed a "client system." It is, in this context, nonetheless acting as a server as it provides graphics services. (And graphics cards are often rather potent "computing engines" these days.)
On the other hand, an X client is typically an application program which must connect to an X Server in order to display windows filled with interesting and useful information.
The client will often run on another host, often a powerful Unix box that would commonly be known as a "server."
The X client might itself also be a "server process" from some other point of view; there is no contradiction here. (Although calling it such may be unwise as it will naturally result in further confusion.)
X nomenclature treats anything that provides display services as an X server. Which is not particularly different from someone saying that a program that provides database services is a database server. Or that a "transaction processor" is a "transaction server."
The upshot (and the point) of all this is that this allows use of the X system that allows processes on various computers on a network to display stuff on display devices elsewhere on the network.
Microsoft has been buying companies and building infrastructure to provide this to their customers using proprietary protocols; X11 has been providing this in more open fashion since 1987.
Material about X and Motif, a long-standing X widget library
MGR is an alternative windowing system that is rather lighter weight than X. It's not a "high performance" system, and is rarely (if ever) used these days. But it had some interesting ideas.
The Joy of X : An Overview of the X Window System; Niall Mansfield; Paperback
The history of X has involved a fair bit of politics. In the "ancient" past, X was developed at MIT as one of the components of Project Athena. It was then managed by the X Consortium. When various Unix things started shifting in various directions, management of the X standard was taken over by The Open Group.
The active ongoing development of X, particularly on free OS platforms, took place through the XFree86 Project, from about the mid '90s until 2003.
In 2003, there were some disputes amongst developers, as well as a controversial change of licensing. Many of the developers have reformed behind a reformed X.org . The publicly stated concerns have typically surrounded the questions: "Why did you change the license, and what exactly does it mean?" It is, however, more realistic to regard this as a controversy over the increasingly closed "governance" of The XFree86 Project Inc. In effect, the concern isn't so much over the details of the license, but rather over the fact that the organization surrounding the software has become way less transparent, and, by their reluctance to explain what they intended by the license changes, have introduced what are really organizational risks that others are choosing to reject.
In 2004, most major Linux distributions opted not to distribute XFree86 4.4, including Red Hat , Mandriva , Debian, and Gentoo , as well as OpenBSD . In 2005, the X.org release of X11 became X server "of choice", as most Linux and BSD switched over in their respective release cycles.
New organization to replace the X committee as part of The Open Group. Membership for smaller organizations is $3000...
Note that this is one of the very few permissible domain names with just one letter. IANA doesn't permit them anymore.