In zero-sum games, the fortunes of the players are inversely related. In tennis, in chess, in boxing, one contestant's gain is the other's loss. In non-zero-sum games, one player's gain needn't be bad news for the other(s). Indeed, in highly non-zero-sum games the players' interests overlap entirely. In 1970, when the three Apollo 13 astronauts were trying to figure out how to get their stranded spaceship back to Earth, they were playing an utterly non-zero-sum game, because the outcome would be either equally good for all of them or equally bad. (It was equally good.) Back in the real world, things are usually not so clear-cut. A merchant and a customer, two members of a legislature, two childhood friends sometimes—but not always—find their interests overlapping. To the extent that their interests do overlap, their relationship is non-zero-sum; the outcome can be win-win or lose-lose, depending on how they play the game.

—from the introductory chapter of Nonzero: The Logic of Human Destiny by the Earthling. For elaboration on non-zero-sum logic, including a discussion of the classic non-zero-sum game "the prisoner's dilemma," see the book's first appendix.