  We determine the basic physical characteristics of eight Gamma-Ray Bursts --
980519, 990123, 990510, 991028, 991216, 000301c, 000926 and 010222 -- by
modelling the broadband emission of their afterglows. We find that the burst
kinetic energies after the GRB phase are well clustered around a mean value of
3E50 ergs. In contrast, the energy release in gamma-rays, after correcting for
collimated explosion, varies among bursts by more than an order of magnitude.
  The jet initial apertures are the 2-14 degree range, mildly correlated with
the energy, half of the jets being narrower than ~3 deg. This implies that,
within 100 Mpc, there are about 10 GRB remnants (expanding at ~0.1c) which can
be resolved with VLBA.
  For all eight afterglows the total energy in the shock-accelerated electrons
is close to equipartition with protons. However the slope of the power-law
electron distribution is not universal, varying between 1.4 and 2.8.
  In at least half of the cases, the density structure of the medium is
inconsistent with an r^{-2} profile. A homogeneous medium with density in the
0.1-50 per cc range can accommodate the broadband emission of all afterglows,
with the exception of 990123, for which we find the density to be less than
0.01 per cc. If GRBs arise from the core collapse of massive stars, then such
low densities indicate the existence of superbubbles created by the supernovae
and winds within a cluster of massive stars.
