I will quote a paragraph from this article http://www.soccerballworld.com/Physics.htm which also explains the phenomenon:
Consider a ball that is spinning about an axis perpendicular to the flow of air across it. The air travels faster relative to the center of the ball where the periphery of the ball is moving in the same direction as the airflow. This reduces the pressure, according to Bernouilli's principle. The opposite effect happens on the other side of the ball, where the air travels slower relative to the center of the ball. There is therefore an imbalance in the forces and the ball deflects - or, as Sir J J Thomson put it in 1910, "the ball follows its nose". This lateral deflection of a ball in flight is generally known as the "Magnus effect".
The forces on a spinning ball that is flying through the air are generally divided into two types: a lift force and a drag force. The lift force is the upwards or sidewards force that is responsible for the Magnus effect. The drag force acts in the opposite direction to the path of the ball.
Let us calculate the forces at work in a well taken free kick. Assuming that the velocity of the ball is 25-30 ms-1 (about 70 mph) and that the spin is about 8-10 revolutions per second, then the lift force turns out to be about 3.5 N. The regulations state that a professional football must have a mass of 410-450 g, which means that it accelerates by about 8 ms-2. And since the ball would be in flight for 1 s over its 30 m trajectory, the lift force could make the ball deviate by as much as 4 m from its normal straight-line course. Enough to trouble any goalkeeper!
It's a very interesting article which explains this effect in detail it's very long though, so I didn't find the use to copy all of it here.
You will also find images there, defining the aerodynamics behind the effect.
This effect also is mentioned in this blog http://scienceblogs.com/dotphysics/2010/09/07/the-curving-soccer-ball