How MUD's Work

From the players' point of view, the underlying mechanisms are of little or no interest. To them, MUAs are environments where things happen. Players have 'personae', which exist in a world elsewhere. The computer is their interface to this otherworld, carrying out their orders and reporting back to them what has happened. MUAs are sprawling landscapes, richly described, and you can try anything (within reason) that you like.

- Richard Bartle, Interactive Multi-User Computer Games, Game Architecture, 21 Jan 1999.