Gamma rays (GRs) are generally described as electromagnetic radiation (EMR) with wavelengths in the range of 0-10 pm. In the EMR spectrum, they include everything beyond X-rays in frequency and photon energy. They are generally the result of nuclear reactions, and are present in cosmic rays. They are often specified by their photon energy in electron volts (eV), as opposed to using their frequency or wavelength. Equivalent to the above-cited wavelength-range "everything beyond 124 keV", but astronomers often cite the line between X-rays and gamma rays as 100 keV (i.e., a round number of eV): this discrepancy is largely moot because discussions of EMR in the 100-124 keV photon-energy range, the energy of interest is likely to be stated.
There is no consensus across all scientists/technologists on the boundary between EMR termed X-rays and gamma rays: the distinction originally arose from the manner in which they are produced: X-rays were from high-voltage vacuum tubes producing cathode rays, and gamma rays are one type of emission occurring during radioactive decay, of radioactive substances such as radium. However, both these types of sources produce EMR, and the wavelength ranges overlap. Mechanisms that produce this short-wavelength EMR don't completely adhere to any particular absolute limits, making any such limits somewhat arbitrary, and in astronomical observation, there will be some particular instrument or source that spans any chosen boundary.
The targets of gamma-ray astronomy include gamma-ray transients (gamma-ray bursts, GRBs) and a gamma-ray background (cosmic gamma ray background as well as a galactic gamma ray background due to the interaction of cosmic rays with the interstellar medium) as well as gamma-ray sources, such as the Crab Nebula. Among the space telescopes/observatories:
Among ground Cherenkov detectors: