A gamma ray was, originally, a photon emitted by a radioactive substance. This has been generalised by physicists to mean any high-energy photon. The energy of gamma rays is the highest of any in the defined electromagnetic spectrum of photon energies. The frequency of gamma ray photons ranges from 3*1019 Hz to 3*1021 Hz. In contrast, ordinary light photons have frequencies from 81*1013 Hz to 162*1013 Hz. Gamma ray frequencies are so large they're cumbersome to handle numerically, so the photon energy (usually in electronvolts) is used instead. In this scale, the frequency range above corresponds to an energy range from 124 keV to 12 MeV.
Other than radioactive decay, gamma rays can be produced by:
There are certainly others...
Since gamma rays have such high energy, they can participate in pair production, where the gamma ray spontaneously produces a particle and its anti-particle, imparting them with all of its energy. These particles are usually electrons and positrons. Often, these particles immediately collide (as they are either neutral or opposite charges) and produce another gamma ray!
Yes, there are entire university physics courses dealing with the behavior of photons.
There are at least three major uses of gamma rays. The most prominent is probably in radiation therapy for cancer treatment. The 'radiation' involved is almost always either gamma rays or high-intensity X-rays. They also have use in astronomy, as only the most energetic objects in the night sky radiate gamma rays in measurable quantities. The Compton Gamma Ray Observatory was the most important astronomica gamma ray instrument before it burned up in 2000. Finally, gamma rays are crucial in the study of subatomic physics, where many important reactions involve the emission of one or more gamma rays. Detecting these gamma rays can reveal important information about the processes that generated them.
This writeup is copyright 2001-2004 D.G. Roberge and is released under the Creative Commons Attribution-NoDerivs-NonCommercial licence. Details can be found at http://creativecommons.org/licenses/by-nd-nc/2.0/ .