We discuss about nuclear emulsion detector, its, history, types, scanning techniques as well as its key role in OPERA experiment. Nuclear emulsion can be considered as analog data storage device from which the automatic scanning system retrieves the data stored in it. The recent improvement of emulsion scanning techniques brings back these nuclear emulsion as one of the most important detectors among all other elementary particle detectors. OPERA uses the emulsion cloud chamber (ECC) technique, which was used previously by the DONUT experiment. As in the OPERA experiment good modularity for large apparatus, the existence of automatic scanning system and availability of fast data scan (Net Scan) are the required parameters, which can only be obtained by utilizing the ECC Nuclear emulsion technique. Therefore, the role of nuclear emulsion is very important in the OPERA experiment.