X windows is the program that draws windows on the screen under most GUI based versions of UNIX. It is important to note that the language that X windows speaks is completely different from that of Microsoft Windows or that which Mac OS X uses.
Any X window system consists of 2 distinct parts - the X server and 1 or more X clients.
It is important to realize the distinction between the server and the client....
The X windows server runs on the machine to which the monitor is connected to. The clients may also run on this machine, communicating directly with the server. On most workstations, this is the normal situation. However, X is a networked window system, and it is possible for the client to run on a remote machine, communicating with the server via some form of network.
It is possible to connect to one of the schools servers and launch graphical applications from one of the UNIX servers. This does require that an X windows server, be installed on the machine from which you are connecting from. This is automatically running if you are running a GUI UNIX system. You can also install an X windows server on top of Microsoft windows, so that you can open up windows locally that are talking to the server at UMBC. Like I said earlier, this does require installing some software on the client side, and time permitting we may talk about this later in the course.
The roll of the Desktop Manager is to provide you with the ability to manage all of the details of your system that would otherwise require you to type in a bunch of commands in a terminal window. These details include managing your files, launching programs, configuring various aspects of your system among many other things.
Gnome and KDE are examples of desktop managers. Both of these look a lot like Microsoft windows. They have the equivalent of a Start Menu, have an equivalent of Windows Explorer, have some soft of control panel.
It is also worthy to note that the desktop manager is optional. Many older systems did not have a desktop manger that sat in the in-between the X server and the Window manager.
The Window Manager is a program that manages the placement of Windows on your system. The Window Manager makes it possible to move, resize, and minimize the various programs running on your computer
I like to think of the Window Manager as the framing around the windows as well as all of the associated functionality that they provide. For example, most all window managers can close, minimize, maximize & resize (such ss Microsoft Windows). However most UNIX window managers add so much more in the way of functionality. The decoration and customization of these windows under UNIX tends to be much more flexible. Many Window Managers also provide other functionality such as Shading/Un-shading, Sticky/Un-Sticky, window history, and desktop and workspace manipulations.
As I mentioned most UNIX systems can be installed without the GUI, and that the GUI is just another application that runs on top of the operating system.
There are many implementations out there of all 3 of these components. It is possible to mix and match implementation and versions of these. They need not be alike and need not be all by the same organization. This is quite a sift in paradigm from Microsoft and Apple.