Stimulated Emission
The word "laser" is an acronym for Light Amplification by Stimulated Emission of Radiation. The easiest laser model to understand is the two level system. In a two level system, the particles have only two availible energy levels, separated by some energy difference which is typically referred to in terms of the photon energy, hv0. These two levels are generally referred to as the upper and lower laser states. When a particle in the upper state interacts with a photon matching the energy separation of the levels, the particle may decay, emitting another photon with the same phase and frequency as the incident photon. Thus we have gotten two photons for the price of one. This process is known as stimulated emission. Population Inversion
A fundamental concept in lasers is the idea of a "population inversion". A normal thermal population in any material will have most of the particles in the ground state. However, we would prefer to have most of the particles in the excited state so we can get free photons through stimulated emission. Thus in a laser we strive to create a "population inversion" where most or all of the particles are in the excited state. This is achieved by adding energy to the laser medium (usually from an electrical discharge or an optical source such as another laser or a flashlamp); this process is called pumping . Gain
Another fundamental concept in lasers is the idea of gain, which is basically a short way of referring to the "free" photons described earlier. Suppose we have just pumped our laser medium so that all of the particles are in their excited state. One of those particles now spontaneously decays back down to its ground state, emitting a photon (hv0). This photon is of the right frequency to stimulate emission from another excited state particle, which emits another photon which can stimulate another excited state particle, and so on.