LASER is an acronym for "Light Amplification by Stimulated Emission of Radiation."
Electrons in a material are pumped up into a higher energy level, causing a population inversion. Existing radiation then stimulates emission of more light with the same frequency characteristics and propagation vector. Light is "stored" in a cavity, with mirrors at each end. Output is often achieved by using a mirror which allows a small amount of transmission at one end of the cavity.
Laser light is characterised by its coherence and near-monochromaticity.