MUD Object Oriented
MOOs are text-based, virtual reality sites that allow people to connect to the same place at the same time. They are completely unlike conventional chat rooms you might be familiar with in that they allow the manipulation and interaction with cyber-objects in addition to just chatting with other people.
When logging onto a MOO you encounter people who talk by typing, listen by reading, and manipulate objects that describe themselves with words in real time. Parenthetically, I could tell you that MOO stands for MUD Object Oriented ... but then I'd have to define MUD (Multi-User Dimension) ... I could mention that the programming language used in creating MOO Objects is C++-based manipulating a database full inherited objects, rights, and descriptions ... but if you understand `a bunch of people and things mediated by text' you get the idea.