It is customary for the author to gloat over the language he or she created in this section, so let's assume that I've said "The TMD language is the single most overwhelmingly and terrifyingly powerful device ever developed by all humankind" and we can get to the good stuff :)
TMD stands for TraderMissions Data - it's the extention you'll likely see on all the standalone scenarios for TraderMissions. The two other extentions you're likely to run into are TMS (for TraderMissions Save; your saved games) and TML (for TraderMissions pLugin; not TMP for obvious reasons.) The format for all of these is the same, however. Yes, that means you can hack your saved game files relatively easily if you want to. This is encouraged; the more familiar you are with the format, the more likely you are to write a plugin :)
The most recent version of this documentation can be found at the TraderMissions website: TM Website
Essentially, this guide is made for people who want to either add things to the TraderMissions universe or create their own universes using the TM engine. Or just for people who want to know what's going on in their save files. You probably don't need programming experience, though it certainly wouldn't hurt. If you're at all curious about making your own plugins or scenarios, this is the guide for you.
It begins with the syntax of the TMD language in section Syntax. The whole section doesn't involve writing any sort of functional plugin whatsoever, but it's necessary so you know what you'll be looking at, and it introduces the fine tmdverify tool. Section Simple Changes contains ways to make simple changes and introduces one of the fundamental rules of TraderMissions. Very simple plugins are covered in this section. More complex plugins are covered in section Complex Changes, with the 'remove' and 'replace' directives being introduced. Finally, section Tutorial is a bottom-up guide to building a fully-featured scenario. The final sections of the guide, Reference and Internal Reference, serve as a reference to the various properties of the objects you'll encounter.
This guide is for version 0.9.0 of TraderMissions - differences between 0.9 and earlier versions are noted where encountered.