  We present an overview of selected aspects of ultrarelativistic
nucleus-nucleus collisions, a research program devoted to the study of strongly
interacting matter at high energy densities and in particular to the
characterization of the quark-gluon plasma (QGP). The basic features of the
phase diagram of nuclear matter, as currently understood theoretically, are
discussed. The experimental program, carried out over a broad energy domain at
various accelerators, is briefly reviewed, with an emphasis on the global
characterization of nucleus-nucleus collisions. Two particular aspects are
treated in more detail: i) the application of statistical models to a
phenomenological description of particle production and the information it
provides on the phase diagram; ii) the production of hadrons carrying charm
quarks as messengers from the QGP phase.
