Pb(Fe0.5Nb0.5)O-3 (PFN), one of the few relaxor multiferroic systems, has a G-type antiferromagnetic transition at T-N = 143K and a ferroelectric transition at T-C = 385 K. By using high-resolution neutron-diffraction experiments and a total scattering technique, we paint a comprehensive picture of the long-and short-range structures of PFN: (i) a clear sign of short-range structural correlation above T-C, (ii) no sign of the negative thermal expansion behavior reported in a previous study, and (iii) clearest evidence thus far of magnetoelectric coupling below T-N. We conclude that at the heart of the unusual relaxor multiferroic behavior lies the disorder between Fe3+ and Nb5+ atoms. We argue that this disorder gives rise to short-range structural correlations arising from O disorder in addition to Pb displacement.