What is the difference between Variance and Covariance?

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Could someone please explain it to me what is the difference between Variance and Covariance?
 
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"Variance" applies to a single random variable. "Covariance" applies to two or more related random variables.
 
Could you please give me an example?
 
DUET said:
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.
 

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