Correlation measures the relationship between two variables. It can be positive, meaning the variables increase or decrease together, or negative, meaning one variable increases as the other decreases. The Pearson correlation coefficient ranges from -1 to 1, with -1 indicating a perfect negative correlation, 0 indicating no correlation, and 1 indicating a perfect positive correlation. Values between -1 and 0 represent negative correlation, while values between 0 and 1 represent positive correlation. Graphically, correlation strength can be classified as small, medium, or large depending on how close the coefficient is to -1 or 1.