  This article reviews several important results from RHIC experiments and
discusses their implications. They were obtained in a unique environment for
studying QCD matter at temperatures and densities that exceed the limits
wherein hadrons can exist as individual entities and raises to prominence the
quark-gluon degrees of freedom. These findings are supported by major
experimental observations via measuring of the bulk properties of particle
production, particle ratios and chemical freeze-out conditions, and elliptic
flow; followed by hard probe measurements. The results reveal that a dense,
strongly-interacting medium is created in central Au+Au collisions at 200 GeV
at RHIC. This revelation of a new state of nuclear matter has also been
observed in measurements at the LHC. Further, the IP-Glasma model coupled with
viscous hydrodynamic models, which assumes the formation of a QGP, reproduces
well the experimental flow results from Au+Au at 200 GeV. This implies that the
fluctuations in the initial geometry state are important and the created medium
behaves as a nearly perfect liquid of nuclear matter because it has an
extraordinarily low ratio of shear viscosity to entropy density. However, these
discoveries are far from being fully understood. Furthermore, recent
experimental results from RHIC and LHC in small p+A, d+Au and He+A collision
systems provide brand new insight into the role of initial and final state
effects. These have proven to be interesting and more surprising than
originally anticipated; and could conceivably shed new light in our
understanding of collective behavior in heavy-ion physics. Accordingly, the
focus of the experiments at both facilities RHIC and the LHC is on detailed
exploration of the properties of this new state of nuclear matter, the QGP.
