...are mostly know as the waves that is emitted as a by-product of radioactive processes. They are at the top of the spectrum that humans can reproduce.
From this point forward, waves start to be very dangerous because they interact on the nuclei in atoms.
TO_EDIT:There is no theoretical upper limit to the energies of gamma-ray photons and no lower limit to gamma-ray wavelengths; observed energies presently extend up to a few trillion electron volts—these extremely high-energy photons are produced in astronomical sources through currently unidentified mechanisms
---- few kiloelectronvolts (keV) to approximately 8 megaelectronvolts
( https://en.wikipedia.org/wiki/Gamma_ray )corresponding to the typical energy levels in nuclei with reasonably long lifetimes
A lot of things in space: super-/hypernovas (exploding (massive)stars), supernova remnants (caused by radio active decay), pulsars (fast spinning (600 rps) of huge objects), afterglow (cooling gasses), jets (result of black hole annihilations)
Nuclear power, radioactive material, particle accelerators
To one definition, the gamma ray band starts where EM radiation is due to nuclear events; another just takes an arbitrary wavelength of 10-11 m
This band of the spectrum is most crowded with uses for 1001 devices such as:
Mobile phones, satellite communications, radar, TV, radio, medical PET scans..
Interaction with matter:
photoelectric effect, Compton scattering, pair production
Kills or mutates living cells