I suggest you do a similar post on correlation, a term people through around without really understanding it. High correlation means two series tend to move together **relative to their respective means**. You can have two perfectly correlated series that diverge wildly, and two uncorrelated (or perfectly negatively correlated) series that converge. I think people think correlation really means cointegration, despite the fact that pretty much no one other than a statistician has run across cointegration.