  GRBs have remained a puzzle for many high-energy astrophysicists since their
discovery in 1967. With the advent of the X-ray satellites BeppoSAX and
RossiXTE, it has been possible to carry out deep multi-wavelength observations
of the counterparts associated with the long GRBs class just within a few hours
of occurence, thanks to the observation of the fading X-ray emission that
follows the more energetic gamma-ray photons once the GRB event has ended. The
fact that this emission (the afterglow) extends at longer wavelengths, has led
to the discovery optical/IR/radio counterparts in 1997-2000, greatly improving
our understanding of these sources. Now it is widely accepted that GRBs
originate at cosmological distances with energie releases of 10^51-10^53 ergs.
About 25 host galaxies have been detected so far for long-duration GRBs. The
observed afterglow satisfies the predictions of the "standard" relativistic
fireball model, and the central engines that power these extraordinary events
are thought to be the collapse of massive stars rather than the merging of
compact objects as previously also suggested. Short GRBs still remain a mystery
as no counterparts have been detected.
