ACTING
		killer		|		achiever
				|
				|
				|
				|
				|
				|
				|
	PLAYERS	----------------+----------------	WORLD
				|
				|
				|
				|
				|
				|
				|
		socialiser	|		explorer
			   INTERACTING

Players who get more fun out of the game world, rather the people who play in it, are achievers if they also prefer doing things to the game world and are explorers if they prefer doing things with it. Players who prefer the fact that the game world is shared with other players over the fact that it's a world at all are killers if they like doing things to others, and socialisers if they prefer to do things with others (eg. gossip!). The horizontal axis is sometimes labelled people/game, and the vertical active/passive; an older labelling scheme, since dropped, had the horizontal as SHARED world/shared WORLD and the vertical as learning EXPERIENCE/LEARNING experience.

Socialisers are 'hearts', killers are 'clubs', achievers are 'diamonds' and explorers are 'spades', hence the theory's name.

For those who are interested in reading further on this topic, there is a fully-fledged academic quality paper on the subject in issue 1 of the "Journal of MUD Research".

See also stable state.