Certain MUDs, including the one I run, even support an embedded programming language in which a user can describe whole new kinds of behavior for the objects they create. A MUD generally has more than one user connected at a time. All of the connected users are browsing and manipulating the same database and can encounter the new objects created by others. The proximics are undetermined. Everyone is simply present, and there can be no limit to the number of people inside of a given space.
Users can indulge in multiple yet simultaneous activities and conversations.
Due to the nature of the medium there is a varying amount of lag time between player's responses.