  We give a pedagogical introduction to quantum discord. We the discuss the
problem of separation of total correlations in a given quantum state into
entanglement, dissonance, and classical correlations using the concept of
relative entropy as a distance measure of correlations. This allows us to put
all correlations on an equal footing. Entanglement and dissonance, whose
definition is introduced here, jointly belong to what is known as quantum
discord. Our methods are completely applicable for multipartite systems of
arbitrary dimensions. We finally show, using relative entropy, how different
notions of quantum correlations are related to each other. This gives a single
theory that incorporates all correlations, quantum, classical, etc.
