We have obtained optical spectrophotometry of V1974 Cygni (Nova Cygni 1992), the brightest nova since 1975, from day 4 to 540 after visual maximum, which we have combined with ultraviolet spectrophotometry to study the ejecta. We show high quality ultraviolet and optical spectra at each major evolutionary phase, identify the lines present during the nebular phase, and show the temporal development of the emission line fluxes and widths. The optical spectral evolution of this nova, using the CTIO system, was PFe,N,ANe. V1974 Cygni is classified as a "neon nova" since [Ne III] λ3869 and [Ne V] λ3425 were the strongest emission lines during the nebular phase. We determined the physical conditions and elemental abundances of the ejecta during the optically thin phase. Because the densities were high and the shell was not spatially resolved we have developed a completely new method which involves the combination of photoionization models and a multiparameter Monte Carlo fitting of the relative emission-line fluxes. The line spectra from days 300, 400, and 500 were modeled, and the inhomogeneity of the ejected material was simulated by considering large variations in the filling and covering factors, as well as letting the density vary as a function of radius. Enhancements above solar were found for He, N, 0, Ne, and Fe with abundances by (number relative to solar and with 90% confidence uncertainties) of N(He)/N =4.4+9-3, N(N)/NH=282+117-192, N(O)/NH=110+43-67, N(Ne)/NH=250+150-159, and N(Fe)/NH=16+28-12. The large neon abundance can only be explained by the ejection of core material from an ONeMg white dwarf. The resulting helium abundance places the ejected mass at about S X 10-5 Msun. This ejected mass and x-ray studies were used to estimate that the mass of the white dwarf is about 1.3 Msun. Line profiles of Balmer, nebular, and coronal lines, obtained in high resolution during the nebular phase, exhibit nearly identical multicomponent, symmetric sawtooth shapes, indicating the same gas morphology over a wide range of ionization.