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MUD stands for Multi-User Dimension, MOO stands for Multi-user Dimension Object-oriented, and MUSH stands for Multi-User Shell. They (and other less common variations besides) all mean basically the same thing. It's an ISP service that's a little like a chat room, but in addition to the other people present also provides an environment and atmosphere. This key difference tends to make it into a vibrant ISP online community. People usually connect to MUDs via either SSH or Telnet.