The origin of GRBs have been a mystery for almost 30 years. Their sources emit a huge amount of energy on short time scales and the process involves extreme relativistic motion with bulk Lorentz factor of at least a few hundred. In recent years, "afterglow", emission in X-ray, optical, IR, and radio was detected. The afterglow can be measured up to months and even years after the few seconds GRB. We review the theory for the gamma-rays emission and the afterglow and show that it is strongly supported by observations. A recent detection of optical emission simultaneous with the GRB, well agrees with theoretical predictions and further constrains the free parameters of the models. We discuss the evidence that some of the bursts are jets, and discuss the prospects of polarization measurements. We review some of the recent clues for the progenitor of these enigmatic events.