The GNS Theory, as originally developed by Ron Edwards, is a relatively amorphous body of work attempting to create a theory of how role-playing games work. Primarily, GNS Theory holds that participants in role-playing games reinforce each other's behaviour towards ends which can be divided into three categories: Gamist, Narrativist and Simulationist.
Strictly, GNS theory is concerned with players' social interactions, but it has been extrapolated to direct game design, both in and outside the world of RPGs. A game can be classified according to how strongly it encourages or facilitates players...