LASER - Light Amplification by the Stimulated Emission of Radiation. A wonderful and catchy name that was devised for the device, althought it isn't quite accurate. LASERs do not amplify
light at all. The misunderstanding here may come from the fact that the light is conglomerated in the laser cavity
until the light that is bouncing back and forth between the mirrors is in large enough quantity that a noticeable amount passes through the partial mirror
usually at one end of the laser cavity.
The predecesor to the LASER was the MASER which emmitted a beam of microwave energy. The concept that operates a laser, that being the emission of photons from atoms excited by other photons (which happen to be coherent), is closely related to Einstein's nuclear reaction concept, meaning that the photons, like neutrons in atomic fission, end up causing the release of more of their own kind, pushing the effect on.
There are many different sorts of lasers, and many lasing mediums within these categories. There are solid state/semiconductor lasers, gas lasers, dye lasers, and others. The first lasers used rubies as the lasing medium. A flash tube was used to pump the ruby with energy, photons in this case. Later, other lasing mediums and pumping devices were discovered. Gas lasers tend to use electricity to excite the atoms.
Some common gas lasers are Carbon Dioxide and Helium-Neon (HeNe) lasers, which emit light in the infrared and red-orange regions respectively. CO2 lasers are some of the most efficient lasers, yielding around 10% of their input energy as laseer light.
Solid state lasers made of silicon and other semiconductors are the most efficient lasers known as they use small multiples of the wavelengths of light to create the laser cavity little energy is lost.
Dye lasers are an interesting development in lasers. The lasing medium can be in a solid or liquid form as long as it contains the dye with the actual lasing properties. These lasers are usually pumped with other lasers, making them very inefficient. One of the most interesting features of dye lasers is that some of them are tunable to various frequencies of light. Most lasing dyes are poisonous, but there are a couple that are safe for human consumption, and since gelatin, including the sort that we all may enjoy eating, is a suitable substance to hold the dye, it is possible to make your dessert lase.
One of the best resources on the web for information on LASERs is Sam's Laser FAQ and contains a massive amount of information on the subject, including concepts and do it yourself instructions.