Gamma rays (GR) are the most energetic part of the electromagnetic spectrum. But they are such energetic, that the wavelength is usually much shorter than the distances inside the atom, so they act more like a particle than a wave in matter. By convention, a GR is a photon with energy above 105 eV, or equivalently, above a frequency of 1019 Hz.
Also we can split the GR spectrum, in turn, in five regimes: low energy range (below 108 eV), high energy range (up to 1011 eV), very high energy range (up to 1014 eV), ultra-high energy range (up to 1017 eV) and extremely high energy range (above 1017 eV).
Observations from low and high energy ranges (below 1011 eV) are called the “spaceborn gamma-ray astronomy”, due to the fact that GR of this energies cannot penetrate the Earth’s atmosphere, so the observations can only be performed from space satellite experiments. On the contrary, the very high and extremely high energy ranges are called the “ground-based gamma-ray astronomy”, because they provoke electromagnetic cascades in the atmosphere producing Cherenkov light which can be detected by optical telescopes on ground.
Due to the fact GR are neutral, their trajectories are not deflected by the galactic magnetic fields, so they travel in straight lines and their sources are relatively easy to determine. In fact, the most of the compact and energetic objects emit GR, like neutron stars, massive black holes, supernova explosions and even the Cosmic Rays (CR) when interacting with the photon background of the universe. However, despite their relatively abundance, during their propagation they interact with photons of lower energies from the Diffuse Extragalactic Background Radiation that permeates the universe (mostly CMB and EBL).
When the GR interact with the photons of the background radiation, they can annihilate producing a pair electron-positron. As the particles in the final state are massive, fixing the energy of the low energy photon (depending of the background) this process has a threshold energy, that implies a strong suppression of the flux of GR above that threshold.
The opacity of the universe to GR can be used to test some predictions of some theories of new physics. And also the sources of ultra high energy GR are usually sources of other messengers also, like CR or neutrinos, which is very important for a multimessenger approach.