  Presently there are several classes of detected gamma-ray extragalatic
sources. They are mostly associated to active galactic nuclei (AGN) and (at
soft gamma rays) to gamma-ray bursts (GRB), but not only. Active galactic
nuclei consist of accreting supermassive black holes hosted by a galaxy that
present in some cases powerful relativistic jet activity. These sources, which
have been studied in gamma rays for several decades, are probably the most
energetic astrophysical objects, and their appearance depends much on whether
their jets point to us. Gamma-ray bursts, thought to be associated to
collapsing or merging stellar-mass objects at cosmological distances, are also
accreting highly relativistic jet sources that shine strongly at high energies.
These are very short-duration events, but they are also the most luminous.
Recently, star formation galaxies have turned out to be also gamma-ray
emitters. On the other hand, clusters of galaxies have not been detected beyond
X-rays yet. These are the largest known structures in the Universe; in their
formation through accretion and merging, shocks and turbulence are generated,
which may lead to gamma-ray production. In this work, the gamma-ray physics of
AGNs is briefly presented, as well as that of starburst galaxies, GRBs and
clusters of galaxies. Afterwards, we consider some particular cases of
gamma-ray production in non-blazar AGN jets interacting with their medium at
different scales.
