Correlation and Causality

As we wrapped up a group discussion I was reminded about the wonderful discussion of correlation and causality from Freakonomics. How do they differ? When is it one, and when is it the other? Why do people confuse them? The book illustrates eloquently the differences between them. And any mention of causality needs the obligatory reference to the Merovingian: You see there is only one constant. One universal. It is the only real truth. Causality. Action, reaction. Cause and effect.