What makes the ball curve: Soccer players can make the ball curve by applying a Force, kicking, to the ball that is not in the center of the ball itself.  When the ball is struck on the side by a player the ball spins while it is moving forward.  In the case of the picture below, the soccer ball was struck on the right side of the ball and is spinning counter-clockwise.  What causes the ball to actually curve in the air is a difference in the pressures on either side of the soccer ball.  On the left side of this soccer ball, the air is moving faster, than the right side, relative to the center of the ball.  This causes a lower pressure to develop on the left side of the ball, while on the right side there is a higher pressure because the air flow is moving slower relative to the center of the ball.  This difference in air pressure causes the ball to curve to the left during its flight path.  This curve is known as the Magnus Effect after the physicist Gustav Magnus.