MUDs (Multi User Dungeons) were among the first online RPG's ever created. They consist mostly of BBSs (Bulletin Board Servers) that allow members to log in and create characters, email each other, chat, and Role-play in real-time. Imagine a game like Ultima Oline or EverQuest, but with text only. It's simple enough that any computer can run them, but complex enough for the most hardcore RPGer. Except that the computer does the math for you. ; )
I'm not into MUDs myself, but they are really popular with alot of people at my university (I hadn't even heard of them until I came here almost three years ago). There are alot of people who are into them, and they are REALLY into them. They spend hours upon hours playing, and they can't stop saying good things about them.
So, while the "rest" of us cling to our polygon-laiden, colored lighting heavy, 3d accelerated games on our big expensive PC's and consoles, the MUDers click away on their keyboards and and have just as much fun. From what I hear there are thousands of MUDs, mostly with AD&D/Fantasy type themes, but there are ones for every occasion, even star wars.
[This message has been edited by Darth Kurgan (edited February 22, 2000).]