Soccer players kick the ball in a linear kick, though you find it to turn sideways, not even in one direction. Just mid air it changes that curve's direction, i.e., it swings, as footballers say. Is there any physical explanation?
Maybe this video will better emphasize this phenomenon: http://www.youtube.com/watch?feature=player_detailpage&v=ZEv7QEFNVq0#t=265s
Best Answer
There's an interesting paper that discusses some of the physics/maths involved in the spiral path of a football. Here's Roberto Carlos' goal against France (discussed in comments to question).
From: Football curves - Journal of Fluids and Structures
Update:
Another article "Gaining insight about the unpredictable movement of knuckleballs by dropping beads into water" describes the so-called Knuckleball shot popularised by Ronaldo.