Several sets of (x, y) points, with the Pearson correlation coefficient r of x and y 
for each set. 

Note that the correlation reflects the noisiness and direction of a 
linear relationship (top row), 
but not the slope of that relationship (middle), 
nor many aspects of nonlinear relationships (bottom). 

N.B.: the figure in the center has a slope of 0 but in that case the correlation 
coefficient is undefined because the variance of Y is zero.

copied from wikipedia