Extensive experimental data from high-energy nucleus-nucleus collisions were recorded using the PHENIX detector at the Relativistic Heavy Ion Collider (RHIC). The comprehensive set of measurements from the first three years of RHIC operation includes charged particle multiplicities, transverse energy, yield ratios and spectra of identified hadrons in a wide range of transverse momenta (pτ), elliptic flow, two particle correlations, nonstatistical fluctuations, and suppression of particle production at high pτ. The results are examined with an emphasis on implications for the formation of anew state of dense matter. We find that the state of matter created at RHIC cannot be described in terms of ordinary color neutral hadrons