[ETL]

"A Baseball with 216 raised red stitches hits the air and curves right under the batter, and the batter swings. Why did the ball drop? Why did the batter swing? What exactly happens to the ball as it is thrown?

What happens to the ball depends on what spin was put on it. What causes the ball to curve, slide or stay in a strait pattern? This all has to do with the fact that there is drag force, or air resistance. A curve ball is created when a ball is spinning. The faster flowing air under the ball creates less pressure, which forces the ball to dive or break. Baseball would be a dull game without drag force because there would be no curves, sliders, or knuckle balls."

http://ffden-2.phys.uaf.edu/211_fall2002.web.dir/Jon_Drobnis/Curveball.html
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Yahoo search results for "curve ball physics":
http://search.yahoo.com/search?fr=yhs-avgb-chrome&type=yahoo_avg_hs2-tb-web_chrome_us&p=curve+ball+physics

[ETL]

The effect is apparently similar to how an airplane wing works, in that a difference of air pressure on one side vs another causes the object to move in a particular direction, the one of least resistance.

From Wiki: Airfoil

“The lift on an airfoil is primarily the result of its shape (in particular its camber) and its angle of attack. When either is positive, the resulting flowfield about the airfoil has a higher average velocity on the upper surface than on the lower surface. This velocity difference is necessarily accompanied by a pressure difference, via Bernoulli’s principle for incompressible inviscid flow, which in turn produces the lift force. The lift force can also be related directly to the average top/bottom velocity difference, without invoking the pressure, by using the concept of circulation and the Kutta-Joukowski theorem.”

http://en.wikipedia.org/wiki/Airfoil