  We discuss the differences and analogies of gravitational clustering in
finite and infinite systems. The process of collective, or violent, relaxation
leading to the formation of quasi-stationary states is one of the distinguished
features in the dynamics of self-gravitating systems. This occurs, in different
conditions, both in a finite than in an infinite system, the latter embedded in
a static or in an expanding background. We then discuss, by considering some
simple and paradigmatic examples, the problems related to the definition of a
mean-field approach to gravitational clustering, focusing on role of discrete
fluctuations. The effect of these fluctuations is a basic issue to be clarified
to establish the range of scales and times in which a collision-less
approximation may describe the evolution of a self-gravitating system and for
the theoretical modeling of the non-linear phase.
