MU*s in general are text based multi user environments where players interact and do stuff. It is based on 1970s and 1980s code that evolved over time through the 90s. It is very much a 'command line' experience where you type in commands using a client (there are purpose built clients, but you can use any basic Telnet client if you don't want to upgrade) and things happen. For example, at the MU* I am most familiar with, a very basic command for talking in an Out of Character manner:
You type in:
+os Hey guys, what's going on?
You and your friends at the MU see:
<OOC> Charlie says, "Hey guys, what's going on?"
Multi User Dungeons are games where users log in and team up to fight in dongeon environments where the bad guys are automated. Stats are built up, weapons are acquired, but beyond hacking and slashing and lvling, there's not much to it.
Multi User Shared Hallucinations have more robust code that allows the users to develop things that allow the users to interact with each other and the environment in a way that encourages role playing and 'story telling' rather than purely hacking and slashing in dungeon environments. Of course, there are still elements of MUDs involved, but by and large users are much more free and able to do their own thing withing having to worry about being skilled in using combat code and things like that.
If you have questions because any of that isn't clear, let me know.
Since October 3rd, 2000 | "Quite a thing to live in fear, isn't it? That's what it is to be a slave."