Mucks are a form of multi-user-dungeon, however, mucks allow extensions to the standard server by programming from within the muck, rather than having to mess with the server's code, the coding language is called MUF - (no jokes please) and is not too bad to work with except it runs in reverse polish notation - this gets kind of confusing sometimes where you are struggling with why it doesn't work. There is another language which is really usefull for building more interesting enviroments, this is called MPI.
Differences between a MUCK and a MUD are primarily that you fight your way up in a MUD, in a MUCK fighting isnt the norm, and everyone generally cannot die unless they really upset the people in charge, at which point you would have probably violated the AUP.
Brief terminology
Every player owns themselves, they may create other items, such as another room, commands, programs etc. Often these things are used to express your character or create areas for others.
When you log in, if you struggle for a way around, type 'help' without the quotes, all the standard commands have help, any which I have added (see 'globals') may not as yet, but I am working on it.