  Gamma-ray bursts (GRB) are short and intense bursts of $\sim$100 keV$-$1MeV
photons, usually followed by long-lasting decaying afterglow emission in a wide
range of electromagnetic wavelengths from radio to X-ray and, sometimes, even
to GeV gamma-rays. These emissions are believed to originate from a
relativistic jet, which is driven due to the collapse of special massive stars
and the mergers of compact binaries (i.e., double neutron stars or a neutron
star and a black hole). This chapter first briefly introduces the basic
observational facts of the GRB phenomena, including the prompt emission,
afterglow emission, and host galaxies. Secondly, a general theoretical
understanding of the GRB phenomena is described based on a relativistic jet's
overall dynamical evolution, including the acceleration, propagation, internal
dissipation, and deceleration phases. Here a long-lasting central engine of the
GRBs can substantially influence the dynamical evolution of the jet. In
addition, a supernova/kilonova emission can appear in the optical afterglow of
some nearby GRBs, which can provide an important probe to the nature of the GRB
progenitors. Finally, as luminous cosmological phenomena, it is expected to use
GRBs to probe the early universe and to constrain the cosmological parameters.
