A MUD is a text-only adventure game which allows many people to explore at once.
I played a MUD briefly at university but got regularly slaughtered by super-players and got a bit fed up with it. However, when I was a teenager these were the games I dreamed about creating.
The original MUD was written in 1979 by Roy Trubshaw, an undergraduate at Essex University and when he finished his degree and moved on Richard Bartle took over. The first external players arrived in 1980. In Bartle’s website there is the interesting comment “The first external wizard was Jez, who in those days was just an enthusiastic schoolkid, but now runs his own computer games company.”
The game has evolved, spawned and is still running today. It is also the evolutionary source of games like Ultima Online, Everquest and the others.