Covariance measures how much two random variables vary together relative to each other. If two variables tend to vary in the same direction, then they have a positive covariance. If they tend to vary in opposite directions, then they have a negative covariance.
The covariance between two random variables X and Y, having expected values 
and 
respectively, is as follows:
where E is the operator for the expected value.
If X and Y are completely independent from each other, then they have zero covariance.
Note that if X and Y have covariance zero, they are uncorrelated but are not necessarily independent.