This article provides an introductory review of the physics of quantum spin liquid (QSL) states, focusing on the exotic ground states and low-energy physics of frustrated spin systems. The authors explain why semi-classical approaches fail in the presence of quantum mechanics and introduce alternative methods to address these systems. The article primarily discusses spin 1/2 systems and focuses on resonating valence bond (RVB) states, which are spin-singlet states that can be viewed as an extension of Fermi liquid states to Mott insulators. These states are classified into $SU(2)$, $U(1)$, or $Z_2$ spin liquid states. The article also covers extensions of these states to include spin-orbit coupling and higher spins ($S > 1/2$), as well as other approaches such as matrix product states, projected entangled pair states, and the Kitaev honeycomb model. Experimental progress in realistic materials, including anisotropic triangular lattice systems, kagome lattice systems, and hyperkagome lattice systems, is reviewed and compared with theoretical predictions. The article concludes with a summary of the current status of the field and highlights the importance of understanding QSL states in condensed matter physics.This article provides an introductory review of the physics of quantum spin liquid (QSL) states, focusing on the exotic ground states and low-energy physics of frustrated spin systems. The authors explain why semi-classical approaches fail in the presence of quantum mechanics and introduce alternative methods to address these systems. The article primarily discusses spin 1/2 systems and focuses on resonating valence bond (RVB) states, which are spin-singlet states that can be viewed as an extension of Fermi liquid states to Mott insulators. These states are classified into $SU(2)$, $U(1)$, or $Z_2$ spin liquid states. The article also covers extensions of these states to include spin-orbit coupling and higher spins ($S > 1/2$), as well as other approaches such as matrix product states, projected entangled pair states, and the Kitaev honeycomb model. Experimental progress in realistic materials, including anisotropic triangular lattice systems, kagome lattice systems, and hyperkagome lattice systems, is reviewed and compared with theoretical predictions. The article concludes with a summary of the current status of the field and highlights the importance of understanding QSL states in condensed matter physics.