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.

17 pages, 8 figures, invited plenary talk at the Texas symposium, Heidelberg, December 2010