The game is played on a grid that's 3 squares by 3 squares. You are X, your opponent is O. Players take turns putting their marks in empty squares. The first player to get 3 of her marks in a row (up, down, across, or diagonally) is the winner. When all 9 squares are full, the game is over. If no player has 3 marks in a row, the game ends in a tie.