Adversarial search is a technique used in game playing to determine the best move when facing an opponent who is also trying to maximize their score. It involves searching through possible future game states called a game tree to evaluate the best outcome. The minimax algorithm searches the entire game tree to determine the optimal move by assuming the opponent will make the best counter-move. Alpha-beta pruning improves on minimax by pruning branches that cannot affect the choice of move. Modern game programs use techniques like precomputed databases, sophisticated evaluation functions, and extensive search to defeat human champions at games like checkers, chess, and Othello.