We outline the physical principles of optical quantum memory in atomic ensembles, with the emphasis on protocols that have been successfully implemented experimentally. Among the protocols under consideration, prominence is given to the regularities of quantum memory on photon echo and slow light and to their implementations in crystals doped with rare-earth ions, which have a long quantum coherence lifetime on optical and spin transitions. We analyze the approaches and problems in achieving high efficiency and other practically significant parameters of quantum memory in real experimental conditions. Also discussed are ways to further improve optical quantum memory and the potential of its application in developing an optical quantum repeater.