We present the first comparative study of high- and low-ionization lines in a large sample of active galactic nuclei(AGNs). We report measurements of C IV lambda 1549, He II lambda 1640, and O III] lambda 1663 paired to measurements of H beta, He II lambda 4686, and Fe IIopt in 52 low-redshift AGNs (z less than or similar to 0.8). Ultraviolet archival spectra covering the region of C IV lambda 1549 were obtained with the Faint Object Spectrograph (FOS) on the Hubble Space Telescope (HST). Moderate-resolution optical spectra (approximate to 4-7 Angstrom) were obtained in most cases within two years of the HST observations. We present an atlas of matching C IV lambda 1549 and H beta broad-line profiles. Velocity centroids at different profile heights and other profile properties were measured after removal of narrow-line components and contaminating satellite lines of other elements. Our results add refinements to the ''standard view'' of nebular conditions in the broad-line region (BLR). (1) The average electron density is most probably weighted toward values higher than expected in standard BLR models. The weakness of O III] lambda 1663 and N IV] lambda 1486 relative to He II lambda 1640 suggest that the semiforbidden lines are collisionally de-excited at n(e) greater than or similar to 10(10.5) cm(-3). (2) There is evidence, supporting earlier work, that photoionization via Ly alpha fluorescence plays a major role in the production of Fe IIUV. We identify and discuss briefly the Fe IIUV multiplets in the range 1550-1750 Angstrom. (3) The observational distinction between the narrow- and broad-line regions is blurred by the contribution of a region of intermediate density (10(6) cm(-3) less than or similar to n(e) less than or similar to 10(7) cm(-3)). Best evidence for this emission comes from the [O III] lambda 4363 and C IV lambda 1549 profiles. It is most likely associated with the innermost part of the narrow-line region. Comparisons between the broad-line profile of C IV lambda 1549, H beta and, whenever possible, He II lambda 1640 suggest (1) that radio-loud and radio-quiet AGNs are very different classes of objects that should be treated separately, and (2) that ionization structure varies within the velocity field. The extreme radial velocity emission is characterized by a high-ionization spectrum dominated by C IV lambda 1549 and He II. Several basic results are: (i) C IV lambda 1549 is almost always blueshifted with respect to H beta. (ii) There is a fundamental kinematic difference between radio-loud and radio-quiet objects relative to the rest frame of the AGN. (iii) Radio-loud objects show predominantly redshifted and asymmetric H beta and unshifted symmetric C IV lambda 1549. (iv) Radio-quiet objects show predominantly blueshifted and asymmetric C IV profiles with H beta usually symmetric and unshifted. Correlation analysis of the radio-quiet sample suggests that C IV lambda 1549 EW, C IV lambda 1549 shift/asymmetry, and the strength of Fe IIopt are related. C IV lambda 1549 profiles with large blueshift/asymmetry show lower EW and stronger Fe IIopt emission. The results suggest that C IV lambda 1549 and H beta arise in different velocity/geometry regimes. C IV lambda 1549 may be predominantly emitted in an outflowing bicone of large aperture (similar to 45 degrees), while Fe IIopt is most probably emitted in a strongly flattened geometry (possibly an accretion disk). Bulk H beta emission may come from a less flattened configuration of clouds. In this interpretation, the flux ratio Fe IIopt/H beta(BC) is an aspect indicator for radio-quiet objects. Radio-loud objects are more enigmatic, and the existence of a reliable optical aspect indicator remains uncertain. There is evidence that line profile properties of C IV lambda 1549 and HP are ''coupled'' in the radio-loud sample, suggesting that a common origin for the high- and low-ionization line emission is a possibility. If this is true, however, we do not have a reasonable explanation for the predominance of H beta redshifts and red asymmetries in that population.
