  In this review, we present an up-to-date phenomenological summary of research
developments in the physics of the Quark--Gluon Plasma (QGP). A short
historical perspective and theoretical motivation for this rapidly developing
field of contemporary particle physics is provided. In addition, we introduce
and discuss the role of the quantum chromodynamics (QCD) ground state,
non-perturbative and lattice QCD results on the QGP properties, as well as the
transport models used to make a connection between theory and experiment. The
experimental part presents the selected results on bulk observables, hard and
penetrating probes obtained in the ultra-relativistic heavy-ion experiments
carried out at the Brookhaven National Laboratory Relativistic Heavy Ion
Collider (BNL RHIC) and CERN Super Proton Synchrotron (SPS) and Large Hadron
Collider (LHC) accelerators. We also give a brief overview of new developments
related to the ongoing searches of the QCD critical point and to the
collectivity in small ($p+p$ and $p+A$) systems.
