  Extragalactic relativistic jets are engines able to carry out to large
distances a huge amount of power, not only in the form of radiation, but
especially in the form of kinetic energy of matter and fields. As such, they
can be thought as one of the most efficient engines of Nature, perhaps even
more efficient than accretion. We are starting to disclose these features
through a detailed study of their properties, made possible by the analysis of
the energy band where they emit most of their electromagnetic output, namely
the gamma-ray band. That is why the observations by the Fermi satellite and by
the ground based Cherenkov telescopes are crucial to understand extragalactic
jets. At the start, we believe they are magnetically dominated. And yet, on the
scale where they emit most of their luminosity, their power is already in the
form of kinetic energy of particles. The spectral properties of bright sources
show a trend, controlled mainly by the bolometric apparent luminosity. With
improved sensitivity, and the detection of weaker sources, we can explore the
idea that the spectral trends are a result of the same physical quantities
controlling the emission of non-jetted sources: the black hole mass and the
accretion rate. This is based on recent results on sources showing a thermal
component in their spectrum, besides a non-thermal continuum. That the jet
power should be linked to accretion is intriguing. Most of the apparent
diversity of extragalactic radio sources can then be understood on the basis of
the viewing angle, controlling the relativistic Doppler boosting of the
emission, the black hole mass and the accretion rate.
