Active Galactic Nuclei (AGN) are extragalactic sources, in many cases clearly
associated with nuclei of galaxies (although in the most distant objects the
host galaxy is too faint to be seen), whose emission is dominated by
non-stellar processes in some waveband(s) (typically but not exclusively the
optical). One important feature of AGN is the fact that their emission covers
the whole electromagnetic spectrum, from the radio to the 
-ray band,
sometimes over almost 20 orders of magnitude in frequency.
It is now well established that AGN are strong -ray (E > 100 MeV)
emitters. To be more specific: 1. at least 40% of all EGRET sources are AGN
(Thompson et al. 1995,
1996;
some more AGN are certainly present amongst the
still unidentified sources) and these make up almost 100% of all
extragalactic sources (the only exceptions being the Large Magellanic Cloud
and possibly Centaurus A); 2. all detected AGN are
blazars, that is BL
Lacertae objects (BL Lacs) or flat-spectrum radio quasars
(FSRQ). (1) To appreciate the relevance of
the latter point, we will first have to tackle the subject of AGN
classification.
1 The term ``blazar'' is here given a
wider meaning than the
one sometimes implied, which is restricted to highly polarized quasars (HPQ)
and/or optically violently variable (OVV) quasars. The reason is that there is
increasing evidence that these categories and the flat-spectrum radio quasars,
which reflect different empirical definitions, refer to more or less the same
class of sources. That is, the majority of flat-spectrum radio quasars tend to
show rapid variability and high polarization.