  Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_{3}$ is one of the simplest representatives of the
class of lead relaxors and often serves as a model system for more complicated
compounds. In this paper we analyse both polarized and depolarized Raman
scattering spectra, measured in the temperature range between 1000 and 100 K,
by multiple peak decomposition. Basing on this analysis, we present the picture
of transformations in the crystal and the nature of the spectral lines.
According to our model, the formation of the Fm3m symmetry in the chemically
ordered regions as well as the appearance and freezing of the polar nanoregions
are the consequences of the same phenomenon: ion off-centered displacements and
their reorientational thermal motion. The lowering temperature progressively
limits this motion from eight to four, two and only one site. Raman scattering
exists due to the presence of the Fm3m and polar clusters. It is a product of
the multiphonon interaction process, mediated by the presence of the dynamic
and static disorder.
