A the correlation coefficient, often called Pearson's r or the product-moment correlation, is perhaps the most commonly cited and used measure of linear association or correlation in the social and policy sciences.
Some pictures illustrate the point.
In the case of perfect negative correlation it is -1.0. In this example, the data points are scattered randomly. See Agresti and Finlay page 321 for other examples. So for example, if one is examining the relationship between income and education, r will have the same value regardless if one uses dollars or yen or marks or pesos. For this reason it is called a standardized measure.