Strategies:

Strategies are simply the choices available to the players. If a game has purely simultaneous moves made only once, then each player's  strategy is simply the action taken on that single occasion. But  if a game has sequential moves, then the actions of player who moves later  in  the game can respond  to what other players  (or  she herself)  have done  at  earlier  points.  Therefore,  such players must make a complete plan  of action, for example "if  the other does A, then I will  do X  but  if  the  other does B  then will do Y". This complete plan  of action constitutes the strategy of a game. 

Strategy  is a  complete  plan  of action. "Complete"  in  the sense that  it  should specify  in  detail how  someone should  play  the game. A  simple test whether a strategy is complete or not is to specify how one would play the game in detail - describing  action  in every contingency. In fact,  if  one was write this  all down, hand  it  to  someone else,  and  go on vacation,  the person  acting  as the representative could play  the game just  as one would  have played  it, without ever needing  to  disturb  one's  vacation for instructions  on  how  to  deal  with some situation  one had not  foreseen.  

Suppose there. are  n  players  playing  a  game. The alternative available  to player  1,  from where she can choose her  actions  is called  the strategy space for the  lSt  player. If sl,,  slz, sl3,...  ,  s,, are the alternatives available to player  1, then the strategy space for player 1 is the set S₁ = (S₁₁ S₁₂, S_I3, S_1n)