A Correlation matrix describes correlation among M variables. It is a square symmetrical MxM matrix with the (ij)th element equal to the correlation coefficient r_ij between the (i)th and the (j)th variable. The diagonal elements (correlations of variables with themselves) are always equal to 1.00.
Many methods of multivariate statistical analysis rely on a correlation matrix as the initial data, e.g. principal component analysis, factor analysis, path analysis.