All about Variance and Covariance

To quantify the degree to which the two variables, X and Y, vary together (covary).

It is often important when analyzing x+y or x-y.It is often shows different information compared with its correspondence, variance. It shows whether two variables are related, besides whether the variables change strongly or not( random or not).When covariance is not zero, two variables are related.

reference:

http://www.visualstatistics.net/Visual%20Statistics%20Multimedia/covariance.htm