The correlation is one of the most common and most useful statistics. A correlation is a single number that describes the degree of relationship between two variables. Let's work through an example to show you how this statistic is computed.
Height is measured in inches.
Since the correlation is nothing more than a quantitative estimate of the relationship, we would expect a positive correlation. It means that, in general, higher scores on one variable tend to be paired with higher scores on the other and that lower scores on one variable tend to be paired with lower scores on the other. You should confirm visually that this is generally true in the plot above. Now we're ready to compute the correlation value. You don't need to know how we came up with this formula unless you want to be a statistician.