the correlation of two random variables can be interpreted as a measure of the degree to which one of the variables can be written as a linear function to the other. the correlation is more than just a simple re-scaling of the covariance. while the covariance of x and y may numerically be positive,negative or zero, the correlation always lies in the interval [-1,1]. once again we see that independent random variables have a correlation of zero and hence are described as uncorrelated