A gamma ray or gamma radiation, is penetrating electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of photons in the highest observed range of photon energy. Wikipedia
Gamma-rays are a form of electromagnetic radiation, as are radio waves, infrared radiation, ultraviolet radiation, X-rays and microwaves. Gamma-rays can be used to treat cancer, and gamma-ray bursts are studied by astronomers.
Electromagnetic (EM) radiation is transmitted in waves or particles at different wavelengths and frequencies. This broad range of wavelengths is known as the electromagnetic spectrum. That spectrum is generally divided into seven regions in order of decreasing wavelength and increasing energy and frequency. The common designations are radio waves, microwaves, infrared (IR), visible light, ultraviolet (UV), X-rays and gamma-rays.
Gamma-rays fall in the range of the EM spectrum above soft X-rays. Gamma-rays have frequencies greater than about 1018 cycles per second, or hertz (Hz), and wavelengths of less than 100 picometers (pm), or 4 × 10-9 inches. (A picometer is one-trillionth of a meter.) They occupy the same region of the EM spectrum as hard X-rays. The only difference between them is their source: X-rays are produced by accelerating electrons, whereas gamma-rays are produced by atomic nuclei.