From wikipedia:
This paper and pencil game starts on an empty field of 8×10 boxes with goals of two boxes wide on the baselines. In the beginning, a virtual ball is placed in the center of the field, on the crossing of the paper lines. Players move the ball in turns and aim to place it in the opponents goal.
In one turn, the ball may be moved into one of the eight paper line crossings around it (horizontally, vertically, or diagonally) and a segment from the original position to the new one is drawn to mark the move. The ball may move neither on the game field border nor on the segments marking the past moves but it bounces from them, i.e. a player who moves the ball into a position where there is already an end of a segment or a game field border gets another turn.
The first player to place the ball in the opponents goal wins the game. The game may also end when a player does not have a valid move (he or she loses automatically).