The article introduces the concept of quantum discord as a measure of the quantumness of correlations between two quantum systems. The authors, Harold Ollivier and Wojciech H. Zurek, explain that while two classically identical expressions for mutual information generally differ in quantum systems, this difference defines the quantum discord. They argue that the separability of a density matrix does not guarantee the absence of quantum discord, thus showing that the absence of entanglement does not imply classicality. The authors relate this to the quantum superposition principle and consider the vanishing of discord as a criterion for the preferred effectively classical state, known as pointer states. They provide a detailed mathematical framework for defining quantum discord using mutual information and discuss its implications in the context of quantum measurements and decoherence. The article concludes by highlighting that pointer states are stable and can be monitored without entanglement with the environment, making them a key concept in understanding the quantum-classical boundary.The article introduces the concept of quantum discord as a measure of the quantumness of correlations between two quantum systems. The authors, Harold Ollivier and Wojciech H. Zurek, explain that while two classically identical expressions for mutual information generally differ in quantum systems, this difference defines the quantum discord. They argue that the separability of a density matrix does not guarantee the absence of quantum discord, thus showing that the absence of entanglement does not imply classicality. The authors relate this to the quantum superposition principle and consider the vanishing of discord as a criterion for the preferred effectively classical state, known as pointer states. They provide a detailed mathematical framework for defining quantum discord using mutual information and discuss its implications in the context of quantum measurements and decoherence. The article concludes by highlighting that pointer states are stable and can be monitored without entanglement with the environment, making them a key concept in understanding the quantum-classical boundary.