The origin of GRBs has 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 a bulk Lorentz factor of at least a few hundred. In the last two 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 theories for the γ-ray emission and its afterglow, and show that these are strongly supported by observations. A recent detection of optical emission simultaneous with the GRB agrees well 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.