There are three types of lasers: solid state, gas and liquid. While all work according to the same general principles, they are differentiated on the basis of the medium they employ to create the laser action.
In solid state lasers, an electrical current pumps electrons into the laser medium - typically a semiconductor - exciting electrons that are fixed in the medium. Driven into higher energy states, a condition known as a population inversion, the excited electrons quickly decay back into lower energy states, releasing the excess energy as photons. Carefully positioned mirrors bounce photons hitting them at 90 degree angles back and forth, in turn stimulating other excited electrons to emit photons with identical wavelengths, directions of propagation, and polarizations; this is a process called amplification. Because the mirrors are of unequal reflectivity, the photons are eventually able to escape and their output constitutes the laser action.
The first solid state semiconductor-based lasers were built in 1963. Prior to that, and beginning with the first laser ever built in 1958, solid state lasers were insulator-based, typically using a glass or crystal medium like ruby that was pumped by another non-laser light source to achieve a population inversion. As the technology developed, lasers were used to pump other lasers. Solid state lasers have a variety of medical and industrial applications.
Gas lasers first appeared in 1960. Initially, they used a mixture of helium and neon as their medium, with carbon dioxide coming later. In both cases, a high voltage, high frequency electrical current creates an electrical discharge in a tube containing the gas, leading to a population inversion. Gas lasers can also use more powerful and volatile mediums like hydrogen and fluorine - both are commonly found in rocket fuel - where the combustion of the gasses acts as a pump. Gas lasers are generally the most powerful lasers and frequently mentioned in connection with quixotic military applications, a.k.a., "death rays."
Liquid lasers employ colored compounds carried by a solvent, which are then pumped via other light sources to the point where electrons occupy higher energy levels. A wide range of materials can be used, including copper, chromium, dyes, metallic salts or even jello. With a controlled flow of fluid passing over the pump, liquid lasers are more easily stabilized than other types of lasers, making them useful in isotope separation, measuring, and the manufacture of integrated circuits.