Correlation is a numerical expression that signifies the relationship between two variables.
Correlation allows a researcher to explore this relationship by 'measuring the association' between the variables. Some researchers like to call correlation a 'measure of association' because the correlation coefficient provides the degree of the relationship between the variables. If you are interested in a little deeper explanation, please check out this short paper about correlation . Typically, you need at least interval and ratio data. However, you can run correlation with ordinal level data.