What is Box2d, and how could I use it to make flash games?

I heard that it is a physics engine, and I could use it to simulate the throwing of a ball. What exactly is it, and how would I use it with AS3 and Flash CS5.5? Tutorials???
What is Box2d, and how could I use it to make flash games?
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I heard that it is a physics engine, and I could use it to simulate the throwing of a ball. What exactly is it, and how would I use it with AS3 and Flash CS5.5? Tutorials???