LASERS: the stimulated emission of photons.
LASER BEAM: Three atoms are shown in a line in a highly simplified diagram of a laser. The zigzag at the bottom represents an energy source. When energy is added to the laser (the bright flash) electrons in the atoms absorb some of the energy and move to a higher energy level (the small green electron orbitals become larger yellow orbitals). The electron in the left atom decays back to its original state and in so doing emits a photon (schematically illustrated as a red ovoid containing a yellow sine wave). This photon travels along and when it meets the second (still energised) atom it induces that second atom to drop to its lower energy state. The second atom then releases a photon of its own. This second photon has the same energy (wavelength) as the first photon and the two photons travel along in tandem (same phase and same direction). The third atom is stimulated to release a photon as well.
When an atom is triggered to release a photon by interacting with another photon the process is called stimulated emission. This is basically how a laser works. LASER stands for Light Amplification by the Stimulated Emission of Radiation. This animation only shows three atoms and the photons travel down the axis of the laser. In reality, a laser needs vast numbers of atoms to be energised to create a laser beam. At either end of a laser are mirrors that reflect the photons. After energising the laser, photons are released randomly in all directions. Those few photons that happen to travel down the axis get bounced back and forth between the two mirrors. During their travels, they collect additional photons. Soon, vast numbers of photons are travelling together back and forth along the axis. When released these form the coherent laser beam.