Gamma-ray bursts (GRBs), short and intense bursts of gamma-rays coming from random directions in the sky, has puzzled Astronomers since their serendipitous discovery in the late seventies. GRBs are cosmological, hence the relevant energy released is enormous, making GRBs the brightest explosions in the sky. Longer bursts are associated with star forming regions within dwarf irregular galaxies and with the catastrophic death of massive stars. Short GRBs arise in a variety of host galaxies and are most likely the outcome of binary neutron star mergers. The enormous energy release requires extreme conditions and as such GRBs are natural laboratories for extreme physics ranging from ultra strong magnetic field to extremely energetic particles and the strongest gravitational fields. In spite of a great progress GRB still pose numerous puzzles concerning their inner working and as such are among the most enigmatic objects known today. At the same time, as cosmological powerhouses, GRBs can serve as beacon that shine and light the universe and can be seen up to very high redshifts. Thus GRBs provide a new tool to explore the distant Universe.