The article provides an overview of optical quantum memory, emphasizing its importance in quantum information processing and communication. Quantum memory is crucial for various applications, including quantum computing, long-distance quantum communication using quantum repeaters, precision measurements, and single-photon sources. The performance criteria for quantum memory include worst-case fidelity, average fidelity, efficiency, transfer coefficient, conditional variance, multimode capacity, and storage time. Different mechanisms for optical quantum memory are discussed, such as optical delay lines, cavities, electromagnetically-induced transparency (EIT), photon-echo, and off-resonant Faraday interaction. Each mechanism has its advantages and limitations in terms of storage time, efficiency, and fidelity. The article also reviews recent theoretical and experimental advancements, including the DLCZ protocol for creating long-lived entanglement between atomic ensembles and the controlled reversible inhomogeneous broadening (CRIB) and atomic frequency comb (AFC) approaches for efficient storage and retrieval of quantum states.The article provides an overview of optical quantum memory, emphasizing its importance in quantum information processing and communication. Quantum memory is crucial for various applications, including quantum computing, long-distance quantum communication using quantum repeaters, precision measurements, and single-photon sources. The performance criteria for quantum memory include worst-case fidelity, average fidelity, efficiency, transfer coefficient, conditional variance, multimode capacity, and storage time. Different mechanisms for optical quantum memory are discussed, such as optical delay lines, cavities, electromagnetically-induced transparency (EIT), photon-echo, and off-resonant Faraday interaction. Each mechanism has its advantages and limitations in terms of storage time, efficiency, and fidelity. The article also reviews recent theoretical and experimental advancements, including the DLCZ protocol for creating long-lived entanglement between atomic ensembles and the controlled reversible inhomogeneous broadening (CRIB) and atomic frequency comb (AFC) approaches for efficient storage and retrieval of quantum states.