What Is A MUD
A MUD; originally Multi-User Dungeon, with later variants Multi-User Dimension and Multi-User Domain) is a multiplayer real-time virtual world, usually text-based. MUDs combine elements of role-playing games, hack and slash, player versus player, interactive fiction, and online chat. Players can read or view descriptions of rooms, objects, other players, non-player characters, and actions performed in the virtual world. Players typically interact with each other and the world by typing commands that resemble a natural language.
Read more about the history and structure of MUDs at the article the above is quoted from: