  The discovery of multiband afterglows definitely shows that most $\gamma$-ray
bursts are of cosmological origin. $\gamma$-ray bursts are found to be one of
the most violent explosive phenomena in the Universe, in which astonishing
ultra-relativistic motions are involved. In this article, the multiband
observational characteristics of $\gamma$-ray bursts and their afterglows are
briefly reviewed. The standard model of $\gamma$-ray bursts, i.e. the fireball
model, is described. Emphasis is then put on the importance of the
nonrelativistic phase of afterglows. The concept of deep Newtonian phase is
elaborated. A generic dynamical model that is applicable in both the
relativistic and nonrelativistic phases is introduced. Based on these
elaborations, the overall afterglow behaviors, from the very early stages to
the very late stages, can be conveniently calculated.
