To translate: Multiple User Dungeons were there before Massive Multiple-player Online Role Playing Games. Think you're new with an internet-addiction chatting and hacking away in EverQuest or WoW? In the nineties we already had the text-based MUDs.
And I'll tell, I've known quite a few junkies... Although most MUDs were free, and kept up by many volunteering programmers, flat fee internet was not so common. So, we experienced rather hefty phone bills instead.
What is a MUD? It's a text based adventure mixed in with an online chatbox like MSN or the one on Facebook. Or just imagine an old style adventure like Zork, and connect dozens of players in the same game. That's about how it looks. While you explore the many rooms and chambers of the MUD, you can converse with everyone else online in the environment - and "emote" to your hearts desire. You can do everything up to and including MUDsex... if that's the kind of shared texted fantasy drawing your fancy.
But most of the time MUDders are leveling and building their community, just like regular MMORPGers. Many made friends in far away places, and some eventually met and visited eachothers real countries. So, you can't say MUDs aren't social activity.
Maybe there are less of them than used to be, but MUDs, slightly graphic or more often completely textual still exist. Here you can find some top MUD sites to try.