  Two types of Gamma-ray bursts (GRBs) are observed: short duration hard
spectrum GRBs and long duration soft spectrum GRBs. For many years long GRBs
were the focus of intense research while the lack of observational data limited
the study of short-hard GRBs (SHBs). In 2005 a breakthrough occurred following
the first detections of SHB afterglows, longer wavelength emission that follows
the burst of gamma-rays. Similarly to long GRBs, afterglow detections led to
the identification of SHB host galaxies and measurement of their redshifts.
These observations established that SHBs are cosmological relativistic sources
that, unlike long GRBs, do not originate from the collapse of massive stars,
and therefore constitute a distinct physical phenomenon. One viable model for
SHB origin is the coalescence of compact binary systems (double neutron stars
or a neutron star and a black hole), in which case SHBs are the electromagnetic
counterparts of strong gravitational-wave sources. The theoretical and
observational study of SHBs following the recent pivotal discoveries is
reviewed, along with new theoretical results that are presented here for the
first time.
