Light Amplification by the Stimulated Emission of Radiation
In physics class, land of assumptions, you're taught that atoms' electrons have a specific energy states depending on how many electrons are involved. In reality, these slots are the ground state energy for those electrons. If you give an electron more energy, it will 'switch' to a higher energy (farther away from the nucleus) 'slot'. This is called excitation. Ionization occurs when an electron is excited so much that the nucleus can't hold it anymore.
A LASER device is a (usually cylindical) cavity, filled with a material called the medium, with a complete mirror on one end and a partial mirror on the other. The medium is energized somehow (electrically, chemically, with heat, with light, etc), exciting a large portion of the electrons to a higher state than their ground state. Spontaneously, a photon or a billion are emitted. When that photon moves through another excited atom just right, it stimulates that atom's electron to emit radiation. The emitted radiation (a photon) has the same frequency, phase, and everything else as the original photon. Each of those then stimulates another emission, etc, and a chain reaction occurs. Any photons that are traveling in a path not perpendicular to the mirrors escape. The ones who are perpendicular are reflected and contained. Eventually, the cavity is filled with so many photons, all at the same frequency, all in phase, all bouncing back and forth between the mirrors, that a visible number start are not reflected by the partial mirror, and escape in a coherent beam, a LASER beam.
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