Quantum spin liquids (QSLs) are quantum disordered ground states of spin systems where zero-point fluctuations prevent conventional magnetic order. They are characterized by massive quantum entanglement, leading to unique properties like non-local excitations and topological order. This review discusses QSLs through paradigmatic models and theoretical tools such as gauge theory and partons. It covers different types of QSLs, their models, and experimental probes. The toric code model is highlighted as an example of a topological phase with non-local excitations (anyons) and topological degeneracy. QSLs support exotic quasiparticles with non-trivial mutual statistics, and their entanglement is essential for their properties. Gauge theory is introduced as a framework for understanding QSLs, with U(1) gauge theory providing insights into electric and magnetic charges. The Coulomb phase is discussed, where excitations are non-local and can be created in pairs. The review emphasizes the role of entanglement in QSLs and their connection to topological phases, while also addressing the challenges in experimental study and theoretical classification.Quantum spin liquids (QSLs) are quantum disordered ground states of spin systems where zero-point fluctuations prevent conventional magnetic order. They are characterized by massive quantum entanglement, leading to unique properties like non-local excitations and topological order. This review discusses QSLs through paradigmatic models and theoretical tools such as gauge theory and partons. It covers different types of QSLs, their models, and experimental probes. The toric code model is highlighted as an example of a topological phase with non-local excitations (anyons) and topological degeneracy. QSLs support exotic quasiparticles with non-trivial mutual statistics, and their entanglement is essential for their properties. Gauge theory is introduced as a framework for understanding QSLs, with U(1) gauge theory providing insights into electric and magnetic charges. The Coulomb phase is discussed, where excitations are non-local and can be created in pairs. The review emphasizes the role of entanglement in QSLs and their connection to topological phases, while also addressing the challenges in experimental study and theoretical classification.