Dark Mists(DM) is a fantasy Role Playing (RP) Multi-User Dungeon (MUD)
A MUD is a game in which many people can log in to and interact together. It is somewhat like a chat room(it is real time, and you can talk), but you have an entire world to explore and interact in.
In an RP MUD, you pretend to be another person, basically. When you create a character, you think of the attitudes your character will start with, and act in the MUD according to those attitudes.
Because DM is a fantasy MUD, not all roles are acceptable. For example, a nuclear scientist doesn't really fit into the fantasy/medieval setting (at least I can't see how).