Gamma-Ray Bursts and their jets

Gamma-Ray Bursts are the brightest explosions in the present universe. We observe them as brief flashes of high-energy photons appearing randomly in the sky, followed by a tail of longer wavelengths emission lasting several months. They are associated to the final evolutionary stages of massive stars, and are though to be powered by the formation of a black hole of several solar masses. I will review their properties and discuss the results of numerical simulations of their outflows, the role of magnetic fields and the nature of their spectral evolution.