Basic Statistics - defintions

The covariance of two variables is the average of first variable minus its means times second variable minus its mean
this measures how the variables vary together.

Correlation between two variables is the covariance of the two variables divided by product of standard deviations of the two variables. This standardizes the correlation to lie between -1 to +1
Correlation measures the strength of the linear relationship between two variables.

pg. 46
Bayesian statistics
William m. blostad
wiley