Introduction

In a typical two-player game, you and your opponent alternate turns.  At each turn, you both have exactly opposite goals -- what is best for you is worst for your opponent.  Thus, if you are trying to maximize something, your opponent will try to minimize it.  When you plan a sequence of moves in this type of game, you cannot assume that your opponent will necessarily select moves that will allow your planned sequence.  For example, if you plan to "rob a bank and live the good life", the police might intervene and make the sequence "rob a bank and go to jail".  In a two-player game, you should assume that your opponent will make their best possible move, and you should plan your moves from there.
 

Resources

• For a complete description of game trees, minimax search, and alpha-beta pruning, try this example which discusses the game of Nim.
• If you would like to see just how alpha-beta pruning works, try these animated examples to see a typical case and the best case of alpha-beta pruning for an imaginary game.  For a more detailed text explanation, you may also try this example.