Quantum discord D(A|B) = I(A:B) − J(A:B) measures quantum correlations that go beyond classical correlations. Here I(A:B) = S(A) + S(B) − S(AB) is the quantum mutual information, and J(A:B) = S(A) − min_Π S(A|{Π_k^B}) is the classical mutual information under optimal measurement. Discord is nonzero even for separable (unentangled) states and vanishes only for classically correlated states — making it a finer measure of quantumness. States with nonzero discord enable quantum advantage in certain computation and thermodynamic protocols.