It was a terminal text based D&D where you could connect to a server and create a character and play with other people. Every mud service was a completely different world you could explore and had different characters. You would team up with people and go slay monsters to gain experience and level up or just hang out and do stupid stuff.
You could use a dedicated mud clients like zMud [2] that added some nice key mapping for going north/south/up/down etc. All interaction was text based in CLI.
I was a teenager at that time, late nineties. I spent so many hours playing on a mud called "Merging of Fates" [3] [4] meeting great people day after day. They had occasional meetups in real life, but I didn't grow up in the US so I didn't attend. I think mud was the major factor for improving my English vocabulary as a kid.
I miss those times.
1: https://en.wikipedia.org/wiki/MUD
2: http://www.zuggsoft.com/page.php?file=zmud/zmudinfo.htm
3: http://www.mudconnect.com/mud-bin/adv_search.cgi?Mode=MUD&mu...