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A gamma ray is a high-energy photon, conventionally associated with a transition of an atomic nucleus, radioactive decay or a particle reaction. X-rays and gamma rays are both electromagnetic radiation; their energy ranges overlap, so the production mechanism can be more informative than an arbitrary energy boundary.
A nuclear energy level transition emits a gamma ray whose energy is approximately the level difference, with a correction for nuclear recoil. Detectors infer the incoming energy from interactions such as photoelectric absorption, Compton scattering and, above threshold, pair production. A measured gamma-ray spectrum can therefore identify nuclear transitions even when a detector does not absorb the entire photon energy in each event.
Their energy is very high compared example to more common radiation such as visible spectrum, and there is a neat reason for that: it's because the strong force that binds nuclei is strong so transitions lead to large energy changes.
A decay scheme such as Figure "caesium-137 decay scheme" illustrates well how gamma radiation happens as a byproduct of radioactive decay due to the existence of nuclear isomer.
Gamma rays are pretty cool as they give us insight into the energy levels/different configurations of the nucleus.
They have also been used as early sources of high energy particles for particle physics experiments before the development of particle accelerators, serving a similar purpose to cosmic rays in those early days.
But gamma rays they were more convenient in some cases because you could more easily manage them inside a laboratory rather than have to go climb some bloody mountain or a balloon.
The positron for example was first observed on cosmic rays, but better confirmed in gamma ray experiments by Carl David Anderson.