They are different. Var (X + Y) is like taking the variance of 1 random variable Z which is defined as Z = X + Y. So it is a regular variance.
Covar (X,Y) describes the co-movement between X and Y, whereas X and Y are separate and distinct random variables (they are not combined in any way).
The covar(X,Y) can take any value from negative infinity to positive infinity, so it doesn't inform as much as a standardized covariance, which we call the Pearson Correlation Coefficient, rho, ,which is defined as follows: rho = Covar(X,Y) / (stdev(X)*stdev(Y)).
Rho is defined from -1 to +1. A rho = -1 means perfectly negatively correlated whereas a rho = +1 means perfectly positively correlated.
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