MUDs are a networked service. Clients and servers simply need to be on the same network in order to connect to each other. Thus one need not be logged into the same computer as the people with whom one is communicating. MUDs are multiuser capable. A large number of people can interact with each other at once.
MUDs are extensible.
Any decent MUD server will have an embedded programming language that may be used to extend the database of server objects and to create new commands. If the tool is to be adapted to new uses, it must be flexible. MUDs, in conjunction with most clients, have a history mechanism. Even though the interaction on a MUD happens in real-time, it can be recorded by a client that is connected. Thus one may read a conversation that happened when one wasn't actively involved.