At high temperatures or high baryon number density, QCD describes a world of weakly interacting quarks and gluons very different from the hadronic world in which we live. This raises the possibility of a phase transition as the temperature or density is increased. This subject of phase transition from a state of matter where quarks are confined inside hadrons to one where quarks are free to move around within a large volume - the ''quark-gluon~plasma'' (QGP), is an interesting physics issue. This can be addressed through experimental studies involving relativistic heavy-ion collisions. Lattice gauge theory calculations suggest that critical temperature for such a phase transition is around 150 MeV, corresponding to an energy density of 2-3 GeV/fm3. It has been estimated that the energy density achieved in the central region of nucleus-nucleus collisions could reach as high as 1-10 GeV/fm3, suggesting that such collisions could be used to create matter in the QGP state in the laboratory. This resulted in several generations of experiments at CERN and BNL to search for the formation of QGP at ultra-relativistic energies. The experimental searches were focussed on isolating signatures of two types of phase transitions which might occur in extremely hot and/or dense nuclear matter. One is related to the deconfinement of quarks while the other is related to chiral symmetry restoration. The $''deconfinement''$ phase transition is expected to occur when the hot system of quarks and gluons no longer feel the long range confining force that binds them into hadrons. The other type of phase transition is associated with the restoration of chiral symmetry, corresponding to the melting of ''quark~condensate'' may be found in the ground state of QCD. An illustration of the QCD phase diagram (temperature vs. baryon number density) is shown in phase_diagram.