  Cosmological GRBs are discussed with an emphasis on their plausible
connection with black holes. GRBs can be triggered by collapse of stellar-mass
objects that leads to formation of a black hole and a transient debris disk
with a huge accretion rate. The disk is believed to produce a relativistic jet
(``fireball'') that expands and emits to infinity the observed burst of
gamma-rays. This accretion-jet picture is similar to quasars and X-ray
binaries, however, there are important differences: the physical conditions and
the cooling mechanism in the disk are very different. The observed radiation is
emitted when the expanding fireball becomes transparent, at distances much
larger than the Schwarzschild radius. The burst is then observed as a powerful
relativistic explosion and the transient accretion disk in its center serves as
a brief source of energy that drives the explosion. The explosion picture
depends on the fireball nuclear composition which is shaped close to the black
hole. A large amount of free neutrons survive till the emission phase and link
the physics of the central engine to observed radiation.
