This document describes MUDL, the Multi-User Dungeon Language created by Jason Modisette for use on Sloth MUD. MUDL is a compiled scripting language that can be used from inside of Sloth MUD to add content to the game. The language is not Turing-complete, which means that it's somewhat harder to get into trouble with it than it is with a "real" language. (For example, there's no way to write an infinite loop in MUDL.)
If you are a MUDL programmer who knows the ropes and wants detailed help on something, then you probably want the function reference (or perhaps the calling forms). This link also provides reference information for things that aren't technically functions, like foreach and %a.
There are also several tutorials, as well as a chatty overview of some of the interesting features of MUDL (in the opinion of its author). If you don't want to program in MUDL, but just want to see what it is, I recommend the overview.
If you have another MUD and want to integrate MUDL into it, you can email the author at jpmodisette:AT:gmail:DOT:com. Sloth MUD is a heavily modified DIKU mud, based on the gamma 0.0 code release (so it split from the main DIKU base in about 1991). A glance through the function reference should give you some idea of how much modification it would take to bolt on MUDL to your MUD. I don't promise any help doing this, or indeed that I will even provide MUDL - right now it is solely a feature of Sloth MUD and it's likely to stay that way for the immediate future.