Abstract: Since we entered the 21st century, our understanding of the state of matter at the most fundamental level has been significantly deepened, mainly due to the successful running of the relativistic heavy-ion collider at Brookhaven National Laboratory in the U.S. and the Large Hadron Collider at CERN. In particular, a new state of matter, the so-called quark-gluon plasma,has been extensively investigated, and many new and important achievements have been made.In this article we shall discuss the following points: 1) the strong interaction and phase structure of quantum chromodynamics, 2) the various properties of the quark-gluon plasma, and 3) the experimental signals of the quark-gluon plasma in high energy nuclear collisions.