  By analyzing recent microscopic many-body calculations of few-nucleon systems
and complex nuclei performed by different groups in terms of realistic
nucleon-nucleon (NN) interactions, it is shown that NN short-range correlations
(SRCs) have a universal character, in that the correlation hole that they
produce in nuclei appears to be almost A-independent and similar to the
correlation hole in the deuteron. The correlation hole creates high-momentum
components, missing in a mean-field (MF) description and exhibiting several
scaling properties and a peculiar spin-isospin structure. In particular, the
momentum distribution of a pair of nucleons in spin-isospin state $(ST)=(10)$,
depending upon the pair relative ($k_{rel}$) and center-of-mass (c.m.)
($K_{c.m.}$) momenta, as well as upon the angle $\Theta$ between them, exhibits
a remarkable property: in the region $k_{rel}\gtrsim 2\,fm^{-1}$ and
$K_{c.m.}\lesssim 1\,fm^{-1} $, the relative and c.m. motions are decoupled and
the two-nucleon momentum distribution factorizes into the deuteron momentum
distribution and an A-dependent momentum distribution describing the c.m.
motion of the pair in the medium. The impact of these and other properties of
one- and two-nucleon momentum distributions on various nuclear phenomena, on ab
initio calculations in terms of low-momentum interactions, as well as on
ongoing experimental investigations of SRCs, are briefly commented.
