  The gravitational clustering of collisionless particles in an expanding
universe is modelled using some simple physical ideas. I show that it is indeed
possible to understand the nonlinear clustering in terms of three well defined
regimes: (1) linear regime (2) quasilinear regime which is dominated by
scale-invariant radial infall and (3) nonlinear regime dominated by nonradial
motions and mergers. Modelling each of these regimes separately I show how the
nonlinear two point correlation function can be related to the linear
correlation function in heirarchical models. This analysis leads to results
which are in good agreement with numerical simulations thereby providing an
explanation for numerical results. The ideas presented here will also serve as
a powerful anlytical tool to investigate nonlinear clustering in different
models. Several implications of the result are discussed.
