Correlation is a measure of the linear relationship between two variables. The Pearson Product Moment Correlation Coefficient, represented by r, ranges from +1.00 for a perfect positive relationship to -1.00 for a perfect negative relationship. If there is no linear relationship, r=0. Thus, r indicates both the direction (positive or negative) and the strength of the linear relationship (e.g., weak or strong).
If a relationship is nonlinear, then simple r and regression may not be the appropriate model.