  In high energy nucleus-nucleus collisions, a transient state of thermalized,
hot and dense matter governed by Quantum Chromodynamics is produced. Properties
of this state are reflected in the bulk low transverse momentum (P_T) hadron
production which represent the remnant of the collective medium as well as in
modifications of so-called probes which are not part of the thermalized medium,
i.e. jets generated in high P_T processes or leptons and photons which do not
participate in the strong interaction. Theory effords aim at deducing the
properties of QCD thermodynamics and collectivity from such observables.
