Quantum spin liquids (QLS) are considered "quantum disordered" states of spin systems where zero-point fluctuations prevent conventional magnetic long-range order. These states are characterized by massive many-body entanglement, which gives rise to unique physical properties such as non-local excitations and topological order. The review discusses the nature and properties of QLS based on paradigmatic models and general arguments, introducing theoretical tools like gauge theory and partons. It covers different types of QLS, their models, and the experimental probes used to study them. The focus is on the theoretical aspects, driven by the goal of bridging theory and experiment. The review highlights the importance of entanglement in supporting non-local excitations, such as "spinons," and the role of gauge theory in describing these states. It also discusses the connection between QLS and microscopic spin models through partons and the symmetry and quantum numbers of these states. The review concludes with a summary of the current status of models and experiments for QLS.Quantum spin liquids (QLS) are considered "quantum disordered" states of spin systems where zero-point fluctuations prevent conventional magnetic long-range order. These states are characterized by massive many-body entanglement, which gives rise to unique physical properties such as non-local excitations and topological order. The review discusses the nature and properties of QLS based on paradigmatic models and general arguments, introducing theoretical tools like gauge theory and partons. It covers different types of QLS, their models, and the experimental probes used to study them. The focus is on the theoretical aspects, driven by the goal of bridging theory and experiment. The review highlights the importance of entanglement in supporting non-local excitations, such as "spinons," and the role of gauge theory in describing these states. It also discusses the connection between QLS and microscopic spin models through partons and the symmetry and quantum numbers of these states. The review concludes with a summary of the current status of models and experiments for QLS.