X (or the 'X Window System') is the windowing and graphics environment of choice for Unix systems. Its particular strength (and the key bit that we are interested in for this document) is that it separates the running applications (web browser, word processor, etc) from the actual graphics screen and input devices (mouse, keyboard, etc) via a network communications mechanism.
Essentially, this means that you can be running an application on one machine, but have its input and output redirected to another machine via a network. This is the key feature that makes an X terminal possible.
This document should be treated as a 'getting started with XDM' document, in that it describes the basic terms and concepts for using XDM and X terminals, with simple examples that provide the minimum amount of security.
The reader is advised to consult the list of resources provided at the end of the document in order to proceed beyond these basic facilities - in particular, the configuration of the 'authentication' and security settings should be examined, as the examples given in this document utilise the least secure modes of operation.
Please note - the majority of the information in this document was obtained from systems running Debian 2.1, SuSE 6.4, Mandrake 7.0 and RedHat 6.0.
This document does not discuss the installation or configuration of a network or X on Linux. Please refer to the appropriate HOWTO documents from the Linux Documentation Project for details (see Section 7).
This document also does not attempt to describe how to install and configure Linux for operation as an X terminal. For this information, please refer to the 'thin-client' HOWTO document, provided as part of the Linux Documentation Project, or the Linux Terminal Server Project (see Section 7).
After reading the man pages, specifications and current howto documents relating to XDM and X terminals, I ended up getting really confused about where XDM was supposed to run and confusing XDM servers with X servers and the like, and so after an evening or two of experimentation, this document was born.
Once the basic terminology has been sorted out, the documentation for XDM and self-documenting sample files make very good reading - I just could not find a simple introduction to the basic concepts anywhere to get me started. Hopefully this document could prove to be a suitable introduction to someone in a similar position to me.
Oh, and in case you are wondering, a 486dx2/66 with 16 Mb RAM makes a fine X terminal!