High-energy heavy-ion collisions provide a unique opportunity to study the properties of the hot and dense strongly-interacting system composed of deconfined quarks and gluons – the quark-gluon plasma (QGP) - in a laboratory conditions. The formation of a QGP is predicted by lattice QCD calculations as a crossover transition from hadronic matter (at zero baryochemical potential) and it is expected to take place once the system temperature reaches values above 155 MeV and/or the energy density above 0.5 GeV/fm3. The nature of such a strongly coupled QGP has been linked to the early Universe at some microseconds after the Big Bang. To characterize the physical properties of the short-lived matter (lifetime of about 10 fm/c) experimental studies at Relativistic Heavy-Ion Collider and the Large Hadron collider use auto-generated probes, such as high-energy partons created early in the hadronic collisions, thermally emitted photons, and a set of particle correlations that are sensitive to the collective expansion and the dynamics of the system. The lectures will introduce some of the experimental techniques used within these studies in heavy-ion collisions and provide an overview of the most interesting results. We will also discuss some of the intriguing phenomena found in smaller collision systems that may provide additional insight into the inner workings of the hot QCD matter.