http://en.wikipedia.org/wiki/MOOBut trust me, they're a 10th of the fun you could have with MUSH's.
Here's the break down:
MUD/MOO: Akin to a text based video game, the code is usually C/C++, and it's very restrictive. The only people who can code are those with shell access to the database.
-Example: LambdaMOO, POO, MOOP, and the grandfather: TinyMUD
MUX: Kinda a go between. Really, code wise, it's a MUSH acting like a MUD. So technically its a codebase camoing as a codebase camoing as a codebase. Again, as a coder for these things, this stuff is restrictive.
-Example: TinyMUX
MUSH/MUCK: This is where it's at. Lots of different codebases (I personally like TinyMUSH). Easy to use, anyone can code (However bit #1 has the alpha powers), and theres TONS of them for things from a All Flesh Must Be Eaten, to at least 1-3 SR3 MUSH's. Now the thing is, these are all **roleplay** intensive. Meaning you need a background (Usually) to apply a character there. Chargens are all coded, so it's easier than even chargen programs, ect.
-Example: TinyMUSH, PennMUSH, Rhost.
So, to the original question: I'd suggest getting on a Shadowrun MUSH. Now, at this point no one's come up with a viable SR4 MUSH, however, back in the day SR3 mushes were freaking ABUNDANT! And now, there's dredges of em: Shadowrun: Denver, Shadowrun: Seattle, Shadowrun: Germany, and rumors of others still open.
If you want to ask anymore quesitions, go ahead and PM me.