Blazars, radio-loud active galactic nuclei with the relativistic jet closely
aligned with the line of sight, dominate the extragalactic sky observed at
gamma-ray energies, above 100 MeV. We discuss some of the emission properties
of these sources, focusing in particular on the "blazar sequence" and the
interpretative models of the high-energy emission of BL Lac objects.Comment: 8 pages, 4 figures, to appear in the proceedings of the HEPRO II
conference, Buenos Aires, October 26-30 200