The word "laser" is an acronym for Light Amplification by Stimulated Emission of Radiation. It is used both to refer to the device and to the beam of light it emits.
A laser beam is monochromatic (i.e. has only one frequency) and coherent (i.e. photons have the same phase).
A photon in a laser beam, like in other types of light, is emitted because an electron jumps from a higher energy state to a lower energy state. This phenomenon is known as relaxation. The change in energy of the electron is given off in the form of the photon.
Most photon emissions in nature occur spontaneously!
This phenomenon is called spontaneous emission. Spontaneous emission produces photons of varying energies and polarisations which are ejected in random directions.
Another type of emission is stimulated photon emission. This type of emission occurs when an atom transits from a higher state to a lower state due to stimulation by an incoming photon.
Laser beams are the result of stimulated photon emission.
The stimulating photon is not destroyed or changed in this process.
The emitted photon has exactly the same physical characteristics as the stimulating photon, such as energy, polarisation and direction.
This effectively doubles the incoming photon!
A population inversion (referring to the atoms as the population) occurs when there are more atoms in a higher excited state than in a lower one at any given time.
This requires an input of energy into the system from external sources, known as pumping.
A population inversion ensures that stimulated photon emission becomes continuous (so that a laser can keep producing light).
Population inversion also requires that atoms stay in an excited state for a relatively long period of time. Otherwise the rate of de-excitation would be much greater than the excitation caused by "pumping".
This is known as a meta-stable state and only occurs in certain atoms or groups of atoms.
When a laser device is switched on, the system of atoms within it absorbs energy through pumping.
An inverted population is then formed with a majority of the atoms staying in a meta-stable state (a particular energy level where excited atoms tend to reside for a much longer time than other energy levels).
After some time, photons are emitted by spontaneous emission by the excited atoms.
Some of these photons would interact with other excited atoms, causing stimulated emission. A self-amplifying reaction then occurs where the newly emitted photons go on to cause additional stimulated photon emission.
These photons are of the same energy and are coherent (i.e. they have the same direction and polarisation). Parallel mirrors are used to reflect these photons within the gas container to stimulate further emission.
Some of these photons are allowed to escape the gas container through a semi-reflective mirror (i.e. a mirror that allows some light to pass through). These photons form a laser beam.
Spontaneous emission still occurs in the laser but the photons are randomly emitted and are normally outside of the human visible range.