Atoms are made of a central nucleus with orbiting electrons. The nucleus is composed of protons and neutrons, and individual protons and neutrons are composed of quarks and gluons which are bound inside these particles (also called hadrons). Quarks are always observed to be bound in hadronic states, and free quarks have never been observed. However, lattice calculations of Quantum Chromodynamics (QCD) indicate that at high temperature and pressure, the hadrons essentially melt and the quarks and gluons are asymptotically free. The formation and experimental detection of such a state (called the quark-gluon plasma or QGP) is the primary goal of high-energy nuclear physics.
In lower energy nuclear reactions, the nuclei exchange protons and neutrons. But, at highly relativistic energies the nuclei are destroyed leaving a region in space with an extremely large energy density. This region may be characterized as a quark-gluon plasma.
In the hot reaction region, we are looking for a phase transition of nuclear matter as shown in the above phase diagram. Eventually the system expands and cools, thus crossing back over the phase boundary and binding all the quarks and gluons back into hadrons. By studying the final particle yields, we hope to understand the nature of this phase transition. Our current model of the early universe suggest that it cooled through the quark-gluon plasma phase transition a few microseconds after the Big Bang; the aim of relativistic heavy ion physics is to replicate millions of microscopic versions of this transition, and through them learn more about the nature of the transition