A person, usually a young teenage boy, althought can be anyone, who plays a type of MUD game (Multi-User Dungeon). These are all text based games, and so, the person who would be lame enough to do this, has NO life, hence a mudder.
Tom: You play any internet games yo?
James: Sure, I play this text-based game online called MajorMUD.
Tom: That's lame as hell dude. You mudder.