  This paper describes the conclusions that can be drawn from the data taken
thus far with the PHOBOS detector at RHIC. In the most central Au+Au collisions
at the highest beam energy, evidence is found for the formation of a very high
energy density system whose description in terms of simple hadronic degrees of
freedom is inappropriate. Furthermore, the constituents of this novel system
are found to undergo a significant level of interaction. The properties of
particle production at RHIC energies are shown to follow a number of simple
scaling behaviors, some of which continue trends found at lower energies or in
simpler systems. As a function of centrality, the total number of charged
particles scales with the number of participating nucleons. When comparing
Au+Au at different centralities, the dependence of the yield on the number of
participants at higher pT (~4 GeV/c) is very similar to that at low transverse
momentum. The measured values of charged particle pseudorapidity density and
elliptic flow were found to be independent of energy over a broad range of
pseudorapidities when effectively viewed in the rest frame of one of the
colliding nuclei, a property we describe as "extended longitudinal scaling''.
Finally, the centrality and energy dependences of several observables were
found to factorize to a surprising degree.
