Active Galactic Nuclei and their demography through cosmic time
  Active Galactic Nuclei (AGN) are highly energetic astrophysical sources
powered by accretion onto supermassive black holes in galaxies, which present
unique observational signatures covering the full electromagnetic spectrum (and
more) over about twenty orders of magnitude in frequency. We first review the
main AGN properties and diversities and show that they can be explained by a
small number of parameters. We then discuss the so-called Unification Models
for non-jetted AGN, according to which these sources are believed to have the
same nuclear engine and circumnuclear matter, with the same geometry for the
obscuring structure. This simplified scenario, however, cannot explain all the
observed complexities, such as the presence of multiple absorbers on different
physical scales, including recent X-ray observations of circumnuclear matter.
Finally, we touch upon AGN evolution in the X-ray and $\gamma$-ray bands.
