  The purpose of this short tutorial paper is to review various criteria that
have been used to characterize the quantum character of correlations in optical
systems, such as "gemellity", QND correlation, intrication, EPR correlation and
Bell correlation, to discuss and compare them. This discussion, restricted to
the case of measurements of continuous optical variables, includes also an
extension of known criteria for "twin beams" to the case of imbalanced
correlations.
