General Properties of Jets from Active Galactic Nuclei and Comparison with Protostellar Jets

We present the general properties of the Active Galactic Nuclei (AGNs) and discuss the origin and structure of jets that are associated to a fraction of these objects. In particular, we address the problems of jet acceleration, its possible connection with accretion processes and jet composition. Moreover, we consider the long standing question about the origin of the dichotomy, both in morphology and power, in Fanaroff-Riley I and II radio galaxies suggesting that FR I jets are lighter than FR II ones and therefore more sensitive to the effects of shear-layer instabilities due to the interaction of the jet with the ambient medium. This hypothesis is supported by hydrodynamical numerical simulations in the relativistic regime and in three spatial dimensions. Finally, we make comparisons with jets from Young Stellar Objects (YSOs) and argue about the non dichotomic appearance of YSO jets in contrast with AGN jets.