  Gamma-ray bursts GRBs are among the most mysterious astronomical phenomenon
ever discovered. Unlike most astronomical discoveries which were explained
within weeks or months after their initial discovery, GRBs remain a puzzle for
more than thirty years. During the last decade our understanding of GRBs has
undergone two major revolutions. First, BATSE discovered that GRBs are
distributed isotropically over the sky and thereby demonstrated their
cosmological origin. The second revolution tool place more recently when
BeppoSAX discovered GRB afterglow. This confirmed the fireball model and led to
a wealth of observational data, some of which has not been fully understood
yet. The emerging picture is that GRBs are the most luminous objects and the
most relativistic objects ever discovered: (i) GRBs involve relativistic motion
at a velocity of 0.9999c or larger. (ii) Most current GRB models involve the
formation of a black hole in one way or another. (iii) If binary neutron star
mergers are the sources of GRBs then GRBs are also associated with
gravitational radiation signals. Finally, (iv) as cosmological power-houses
that are observed to high red-shift GRBs might be used to measure cosmological
parameters and to teach us about the epoch of galaxy formation.
