What is the difference between Variance and Covariance?

[DUET]

Could someone please explain it to me what is the difference between Variance and Covariance?

 

[HallsofIvy]

"Variance" applies to a single random variable. "Covariance" applies to two or more related random variables.

 

[DUET]

Could you please give me an example?

 

[mathman]

Could you please give me an example?

Let X and Y be random variables.

var(X) = E(X²) - E(X)²

cov(XY) = E(XY) - E(X)E(Y)

E(..) means average.