Introduction to extragalactic sources of very high-energy photons
  The launch of the Fermi gamma-ray space telescope and the imaging air
Cerenkov telescopes H.E.S.S., MAGIC, and VERITAS have substantially transformed
our knowledge of gamma-ray sources in the last decade. The extragalactic
gamma-ray sky is teeming with blazars, which are active galactic nuclei whose
jet is directed at us. Additionally, there are radio galaxies, starburst and
spiral galaxies, and gamma-ray bursts, albeit with smaller numbers. Galaxy
clusters have not yet been observed in gamma rays. Here, I will introduce the
different gamma-ray emission processes and review what they may tell us about
these objects and the underlying acceleration mechanisms. Beyond the study of
these fascinating objects, TeV gamma rays from blazars probe the integrated
star formation history of the universe. Studies of TeV blazar spectra may
provide us with insights into intergalactic magnetic fields or alternatively,
may lead us to infer the existence of a novel mechanism that heats the
intergalactic medium at late times (for redshifts z<3) and impacts the
Lyman-alpha forest and late-time structure formation. The TeV gamma-ray
emission may also allow us to probe fundamental physics such as the structure
of space time.
