Gamma Ray Bursts: basic facts and ideas

The recent years witnessed a dramatic improvement in our knowledge of the phenomenology and physics of Gamma Ray Bursts (GRBs). However, our ``pillars of knowledge'' remain a few, while many aspects remain obscure and not understood. There is no general agreement on the radiation mechanism of the prompt emission, nor on the process able to convert the bulk motion of the fireball into random energy of the emitting leptons. The afterglow phase can now be studied at very early phases, showing an unforeseen phenomenology, still to be understood. In this context, the detection of ~GeV emission from ~10% of GRBs, made possible by the Fermi satellite, can hopefully shed light on some controversial issues.