MUDs are a class of games played on the net that preceeded MMORPGs as you may now know them. MUD is an acronym for M)ulti (U)ser (D)ungeon/Dimension/etc.. and everyone seems to have their own definition of what the 'D' stands for.
Since I have my own mud, I have lots of opinions on this subject, but I'll spare you the rant. If you want to know.. visit my mud and over time you will get an idea of what they are. It would take too long to articulate them here.
I have collected various information over time that I've found helpful, so I'm putting it here.