For more info, please see http://www.physics.usyd.edu.au/~cross

In baseball and cricket the best pitchers and bowlers know how to make the ball move due to the effects of aerodynamics. If one side of the ball is rough, the ball swings towards that side because turbulent air 'clings' to that side of the ball and is deflected. Although baseballs and cricketballs appear symmetric, they can be made to fly through the air with a smooth or rough side by judicious angling of the seams combined with the axis of rotation.