We present a library of mean ultraviolet stellar energy distributions derived from International Ultraviolet Explorer (IUE) low-resolution spectrophotometry of 216 stars. The library is intended to facilitate interpretation of the composite UV light of stellar populations such as star clusters and galaxies. The spectra cover 1205-3185 Angstroms with a spectral resolution of approximately 6 A. The individual stellar spectra were corrected for interstellar extinction (using the Savage-Mathis law), converted to a common flux scale, and interpolated to a common wavelength scale. Stars were combined into standard groups according to their intrinsic continuum colors, observed UV spectral morphology, MK luminosity class, and metal abundance. The library consists of 56 groups: 21 dwarf (V), 8 subgiant (IV), 16 giant (III), and 11 supergiant (I+II) groups, covering O3-M4 spectral types. A metal-poor sequence is included, containing four dwarf and two giant groups, as is a metal-enhanced sequence with a single dwarf, subgiant, and giant group. More information on the library compilation and descriptions of the behaviour of spectral indices characterizing the continuum and strong absorption features are given in the paper. The spectra themselves (in the uvgroups.fit file) were previously released in the AAS CD-ROM Series Volume VII (1996); see Leitherer et al. "A Database for Galaxy Evolution Modeling" (1996PASP..108..996L). Introduction: Each mean group spectral energy distribution (SED) is a weighted average of good quality, extinction-corrected, normalized IUE spectra for a number of stars (ranging from 2 to 12). Assignments to groups were based on actual UV SED properties, rather than optical-band spectral classifications, and were adjusted to minimize the dispersion within a group and maximize the difference with adjacent groups. The SEDs are given in units of flux per unit wavelength normalized to the V band, i.e. F({lambda})/<F(V)>, where the mean V-band flux <F(V)>, expressed in units of erg/s/cm2/{AA} (=10mW/m2/nm) was computed as <F(V)> = dexp[-0.4*(V+21.175)]. The mean group SEDs are presented in a FITS image file named uvgroups.fit. This contains a single data array with 57 rows and 1245 columns. The first row contains the 1245 element wavelength vector. The remaining 56 rows contain the individual group SEDs in the order listed in the Group Library Contents ("groups.dat" file). The S/N for stars cooler than about 8000K usually becomes very low in the far-UV in the available spectra. Shortward of a cutoff wavelength (listed in Table 5 of Fanelli et al. 1992), which depends on temperature, we have entered the dummy value 1.0e-20 in the data vectors. Flux measures for cool stars just longward of this range are often of low quality, as will be apparent from increased flux scatter; in a few cases, negative flux values occur in this region. We also present two files containing values for the spectral indices measuring lines and continuum structure which are defined in the paper and summarized in the "Note (G1)" section below. The values in the files are updated from those in Table 7 of the paper in two ways. First, we have used an improved integration scheme to average fluxes in bands; second, we have corrected an error in the tabulation of the continuum colors 2600-V and 3000-V. Owing to a transcription error, the original published values were for narrow bands centered in the continuum windows rather than for averages over them. Because of the rapid changes of flux with wavelength in the UV, the new values are often very different from the original ones.