  We present a detailed review of scaling behavior in the magnetically
underdoped cuprate superconductors (hole dopings less than 0.20) and show that
it reflects the presence of two coupled components throughout this doping
regime: a non-Landau Fermi liquid and a spin liquid whose behavior maps onto
the theoretical Monte Carlo calculations of the 2D Heisenberg model of
localized Cu spins for most of its temperature domain. We use this mapping to
extract the doping dependence of the strength, $f(x)$ of the spin liquid
component and the effective interaction, J_eff(x) between the remnant localized
spins that compose it; we find both decrease linearly with x as the doping
level increases. We discuss the physical origin of pseudogap behavior and
conclude that it is consistent with scenarios in which the both the large
energy gaps found in the normal state and their subsequent superconductivity
are brought about by the coupling between the Fermi liquid quasiparticles and
the spin liquid excitations, and that differences in this coupling between the
1-2-3 and 2-1-4 materials can explain the measured differences in their
superconducting transition temperatures and other properties.
