The basic physics behind a laser was thought up by Einstein (that guy was everywhere!) but the first ones were not actually built until the middle of the 20th century.
Laser is an acronym for Light Amplification via Stimulated Emission of Radiation. "Radiation" just means light, and "Stimulated Emission" is a property of some gases because of Quantum Mechanics.
In QM we know that electrons can only be in certain "orbit" in an atom, more properly called energy levels. When an electron makes a quantum leap from one energy level to another lower level (assuming there is a lower level) It will release a photon (sort of a piece of light) with a very specific frequency. (Frequency and wavelength are inversely proportion, so one frequency corresponds to one wavelength.)
For example in hydrogen if an electron in the second lowest energy level drops down to the very lowest level it will emit a photon with a wavelength of 656.3 nanometers, which turns out to be reddish light. From the third lowest to the lowest the photon wavelength is 486.1 nm, which is blue green.
The point being that an atom can emit light only in very precise frequencies, and not any old frequency.
Einstein developed the concept of "stimulated emission" which is a much more complicated consequence of Quantum Mechanics. It says that if a photon with a transition wavelengh passes close to an excited atom, it will "stimulate" the excited atom to also emit at that frequency.
So in the hydrogen example, lets say the electron is in the 3rd level. If it were to drop to the lowest level the photon would be 486 nm. While the electron is just hanging around on the third level, if some random photon comes flying past with a wavelength of 486 nm this will stimulate the electron to drop and emit its own photon of that frequency. From the outside it looks like one photon went past an atom and then there were two photons.
Therefore the idea in a laser is to get a lot of atoms into an excited state - which is hard, they would rather be in their ground state - and then wait for one of the atoms to emit a photon. This will prompt more atoms to emit photons, and all those will prompt even more to emit photons, etc. etc. The result being "light amplification via the process stimulated emission."
The benefits of a laser include the light being all very nearly at one wavelength, and all the light being in phase.
For a CO2 laser the output light has a wavelength of 10,600 nm, which is in the infrared part of the spectrum and not visible to the human eye. The gas inside a CO2 laser consists of, well, CO2 and also Nitrogen and Helium. The transitions are not simple atomic ones as in the example of hydrogen I gave, but molecular transitions of the CO2 molecule. The basic physics is the same, though.
These other gases are needed as part of the difficult process of getting a lot of the CO2 molecules into an excited state and keep them there long enough so that they will emit photons via the stimulated emission process.