  We discuss the dynamics of parton cascades that develop in dense QCD matter,
and contrast their properties with those of similar cascades of gluon radiation
in vacuum. We argue that such cascades belong to two distinct classes that are
characterized respectively by an increasing or a constant (or decreasing)
branching rate along the cascade. In the former class, of which the BDMPS,
medium-induced, cascade constitutes a typical example, it takes a finite time
to transport a finite amount of energy to very soft quanta, while this time is
essentially infinite in the latter case, to which the DGLAP cascade belongs.
The medium induced cascade is accompanied by a constant flow of energy towards
arbitrary soft modes, leading eventually to the accumulation of the initial
energy of the leading particle at zero energy. It also exhibits scaling
properties akin to wave turbulence. These properties do not show up in the
cascade that develops in vacuum. There, the energy accumulates in the spectrum
at smaller and smaller energy as the cascade develops, but the energy never
flows all the way down to zero energy. Our analysis suggests that the way the
energy is shared among the offsprings of a splitting gluon has little impact on
the qualitative properties of the cascades, provided the kernel that governs
the splittings is not too singular.
