  The ALICE experiment was proposed in 1993, to study strongly interacting
matter at extreme energy densities via a comprehensive investigation of nuclear
collisions at the LHC. Its physics programme initially focused on the
determination of the properties of the Quark-Gluon Plasma (QGP), a deconfined
state of quarks and gluons and was extended along the years, covering a diverse
ensemble of observables related to Quantum Chromodynamics (QCD), the theory of
strong interactions. The experiment has studied Pb-Pb, Xe-Xe, p-Pb and pp
collisions in the multi-TeV energy range, during the Run 1 and Run 2 data
taking periods at the LHC (2009-2018). The aim of this review article is to
gather and summarise a selection of ALICE physics results and to discuss their
implications on the current understanding of the macroscopic and microscopic
properties of strongly interacting matter at the highest temperature reached in
the laboratory. It will be shown that it is possible to have a quantitative
description of the properties of the QGP produced in Pb--Pb collisions. We also
show that various features, commonly ascribed to QGP formation, are detected
for a wide range of interacting system sizes. Precision measurements of
QCD-related observables not directly connected to the study of the QGP will
also be discussed. Prospects for future measurements with the ALICE detector
and its foreseen upgrades will also be briefly described.
