Zero sum games

A game is an interaction or exchange between two (or more) actors, where each actor attempts to optimize a certain variable by choosing his actions (or "moves") towards the other actor in such a way that he could expect a maximum gain, depending on the other's response. One traditionally distinguishes two types of games. Zero-sum games are games where the amount of "winnable goods" (or resources in our terminology) is fixed. Whatever is gained by one actor, is therefore lost by the other actor: the sum of gained (positive) and lost (negative) is zero. This corresponds to a situation of pure competition.

Chess, for example, is a zero-sum game: it is impossible for both players to win (or to lose). Monopoly (if it is not played with the intention of having just one winner) on the other hand, is a non-zero-sum game: all participants can win property from the "bank". In principle, in monopoly, two players could reach an agreement and help each other in gathering a maximum amount from the bank. That is not really the intention of the game, but I hope I have made the distinction clear: in non-zero-sum games the total amount gained is variable, and so both players may win (or lose). When they can both win by cooperating in some way, we might say that their cooperation creates a synergy.

http://pespmc1.vub.ac.be/ZESUGAM.html