A look at the Pearson correlation coefficient (r), the coefficient of determination (r 2), some of their properties and a few examples. (This is discussed in the context of measuring the strength of the linear relationship between 2 variables.)
One of the data sets used is estimated from a figure given in:
Singer et al. (2004). Empathy for pain involves the affective but not sensory
components of pain. Science, 303:1157-1162.