  Radiative feedback (RFB) from stars plays a key role in galaxies, but remains
poorly-understood. We explore this using high-resolution, multi-frequency
radiation-hydrodynamics (RHD) simulations from the Feedback In Realistic
Environments (FIRE) project. We study ultra-faint dwarf through Milky Way mass
scales, including H+He photo-ionization; photo-electric, Lyman Werner, Compton,
and dust heating; and single+multiple scattering radiation pressure (RP). We
compare distinct numerical algorithms: ray-based LEBRON (exact when
optically-thin) and moments-based M1 (exact when optically-thick). The most
important RFB channels on galaxy scales are photo-ionization heating and
single-scattering RP: in all galaxies, most ionizing/far-UV luminosity (~1/2 of
lifetime-integrated bolometric) is absorbed. In dwarfs, the most important
effect is photo-ionization heating from the UV background suppressing
accretion. In MW-mass galaxies, meta-galactic backgrounds have negligible
effects; but local photo-ionization and single-scattering RP contribute to
regulating the galactic star formation efficiency and lowering central
densities. Without some RFB (or other 'rapid' FB), resolved GMCs convert
too-efficiently into stars, making galaxies dominated by hyper-dense, bound
star clusters. This makes star formation more violent and 'bursty' when SNe
explode in these hyper-clustered objects: thus, including RFB 'smoothes' SFHs.
These conclusions are robust to RHD methods, but M1 produces somewhat stronger
effects. Like in previous FIRE simulations, IR multiple-scattering is rare
(negligible in dwarfs, ~10% of RP in massive galaxies): absorption occurs
primarily in 'normal' GMCs with A_v~1.
