What are they and why study them? 

According to our present understanding, hadrons (such as protons and neutrons) are composed of quarks and gluons which interact via the strong force. However, contrary to gravity and electromagnetic forces, the strength of strong interaction increases at large distances making it impossible for us to observe free quarks. Nevertheless, we still can still study their properties by creating conditions of high temperatures and density (by colliding nuclei accelerated to very high speed) at which the quarks get deconfined from the hadrons and form a new phase of matter called the Quark Gluon Plasma. In the talk, we discuss qualitatively the RHIC experiment, detectors used to identify the particles produced in a collision and collective flow as a signature of the Quark Gluon Plasma phase.