The laser is a source of coherent light, operating by stimulated emission within an active medium possessing a non-equilibrium energy level structure. Its operation is explained using three and four-level energy diagrams. The optical resonator, which defines the temporal and spatial properties such as mode structure and coherence of the output beam, is detailed. The fundamental relationships between the laser medium’s properties, the resonator’s geometry, and the resulting radiation parameters are established. Both stable and unstable resonator configurations are analyzed. Finally, various laser types are surveyed alongside their technological applications.