  The overall spectra (SED) of blazars, from radio to gamma-ray energies, seem
to obey well defined trends, with a continuity of properties between blazars of
different classes. To quantify this statement we can either investigate their
observed properties (see Fossati et al., this volume), or try to determine
their intrinsic physical parameters by applying specific models, and trying to
fit their SED. Results of the latter approach are reported here. We applied
simple, one-zone, homogeneous models to all blazars strongly detected in the
gamma-ray band, assuming or not the presence of seed (for the inverse Compton
process) photons produced outside the active region. Our results suggest that
the SEDs of blazars are ruled by the amount of radiative cooling suffered by
the electrons producing most of the emission. In turn, the amount of cooling is
ruled by the amount of the external photon emission, which can be identified
with radiation coming from the broad line clouds. Blazar SEDs are therefore
organized in a sequence: objects with no or very weak emission lines (X-ray
selected BL Lacs, or HBL) are characterized by very high electron energies and
a Compton luminosity of the same order of the synchrotron one. These are TeV
sources. BL Lacertae objects selected in the radio band (or LBL) are
characterized by smaller electron energies, more total power and more line
luminosity, and by a larger ratio of the Compton to synchrotron luminosity.
They are GeV sources. Increasing the total intrinsic power and the line
emission luminosity, we have smaller still electron energies (more cooling) and
greater still Compton to synchrotron power ratio. These are flat spectrum radio
quasars, with a high energy peak located at MeV-GeV energies.
