A MOO (MUD, object-oriented) is a text-based online virtual reality system to which multiple users (players) are connected at the same time.The term MOO is used in two distinct, but related, senses. One is to refer to those programs descended from the original MOOcow server, and the other is to refer to any MUD that uses object-oriented techniques to organize its database of objects, particularly if it does so in a similar fashion to the original MOO or its derivatives.