This method is for example used to amplify light signals traveling through optical fibers in telecommunication networks.

When most people refer to "a laser", however, they mean not a light amplifier, but a light source using the LASER principle.

When light hits atoms or molecules, they can change to a state of higher energy. This energy is taken from the light beam - light is absorbed. However, when an atom or molecule hit by light is already in an energy-rich state, it can release energy in form of additional light, thereby amplifying the light beam. This process is called stimulated emission.

Usually, there are many more atoms/molecules in low-energy states than in high-energy states, and absorption outweighs stimulated emission. There are, however, various methods of achieving "inversion", a state with more high-energy atoms/molecules than low-energy ones. Under these conditions, stimulated emission prevails over absorption, and light is amplified.

Concurrent with absorption and stimulated emission, there's a process called spontaneous emission, where light is emitted randomly from energy-rich atoms or molecules. When amplified using the LASER principle, this light is called super luminescent.

To build a laser light source, one needs at minimum a laser medium, and a way to achieve inversion in order to make it super luminescent. Super luminescent nitrogen lasers are particularly easy to build: since nitrogen accounts for 78% of the air we breathe, one can get away with using air as a laser medium. You can find detailed instructions on how to build such a laser on the web, e.g. at http://spt07.chez.tiscali.fr/Accueil.htm. Inversion in the nitrogen gas is achieved by a high voltage electric discharge; therefore be careful not to electrocute yourself! Also, avoid exposing your eyes or your skin to the laser beam. This type of laser emits very short (a few billionths of a second), but powerful light pulses in the near ultraviolet region of the spectrum.

You can find many more recipes for building lasers on the Web. Nitrogen lasers are about the easiest to build, while other laser types typically require materials that are more difficult to obtain or handle.