Killers focus on winning, rank, and direct peer-to-peer competition. They are engaged by leaderboards and ranks.
Achievers focus on attaining status and achieving preset goals quickly and/or completely. They are engaged by achievements, either determined by the game or themselves.
Explorers focus on exploring and a drive to discover the unknown. These players may end up knowing how the game works and behaves better than the game creators themselves.
Socializers focus on socializing and a drive to develop a network of friends and contacts. They help to spread knowledge and a human feel, and are often involved in the community aspect of the game.