Whether the Players Interest in Total Conflict or Not:

In  simple games  like chess or football, there  is a winner and a loser (ignoring the situation of draw). One player's  gain implies loss of the other player. This motivates  the name zero sum  game  for such situations. More generally, the idea  is that the players' interest  is in total conflict. Such conflicts occur when the  players  are dividing up any fixed amount of possible gain, whether  it  is measured  in  profit, votes or scoops of  ice  cream. As  the available gain need always be exactly equal to zero, the term constant sum  game  is  often substituted for zero sum game. 

Most economic  or social games are  not  zero sum or constant sum. Trade  or economic activity more generally offers scope for deals that benefit everyone. Joint ventures can combine participants' different skills and generate  synergy to produce more than  the sum of what they could have produced individually. But the  interests are not  completely aligned either; the partners can cooperate to create a larger amount of payoff but they might clash while deciding how to share it. 

Most games  in  reality have this tension between conflict and cooperation and many of the most interesting  analyses  in  game theory come from  the need  to handle  it.  By  the following two simple hypothetical  examples we  illustrate constant sum games and variable sum game.