Correlation is the indirect relationship between two things, the things have similar traits though you can have one without the other.
Example
As ice cream sales increase, the rate of drowning deaths increases sharply.
Therefore, ice cream causes drowning.
The drowning is actually occuring because the people are out of shape and overweight. Icecream doesnt mean people are overweight or out of shape, though the people who eat icecream tend to fit that mold.
http://en.wikipedia.org/wiki/Correlation_does_not_imply_causation
