Covariance defines how two variables vary in relation to one another. If both the variables tend to move together then it’s called a positive covariance, whereas if they move inversely then it becomes a negative covariance. The major difference between variance and covariance is the number of variables. Variance deals with one variable whereas covariance deals with two. A real-world example of covariance is in molecular biology where the behavior of certain DNAs is studied using the covariance method.