  Galaxies selected at 170um by the ISO FIRBACK survey represent the brightest
\~10% of the Cosmic Infrared Background. Examining their nature in detail is
therefore crucial for constraining models of galaxy evolution. Here we combine
Spitzer archival data with previous near-IR, far-IR, and sub-mm observations of
a representative sample of 22 FIRBACK galaxies spanning three orders of
magnitude in infrared luminosity. We fit a flexible, multi-component, empirical
SED model of star-forming galaxies designed to model the entire ~1-1000um
wavelength range. The fits are performed with a Markov Chain Monte Carlo (MCMC)
approach, allowing for meaningful uncertainties to be derived. This approach
also highlights degeneracies such as between Td and beta, which we discuss in
detail. From these fits and standard relations we derive: L_IR, L_PAH, SFR,
tau_V, M_star, M_dust, Td, and beta. We look at a variety of correlations
between these and combinations thereof in order to examine the physical nature
of these galaxies. Our conclusions are supplemented by morphological
examination of the sources, and comparison with local samples. We find the bulk
of our sample to be consistent with fairly standard size and mass disk galaxies
with somewhat enhanced star-formation relative to local spirals, but likely not
bona fide starbursts. A few higher-z LIGs and ULIGs are also present, but
contrary to expectation, they are weak mid-IR emitters and overall are
consistent with star-formation over an extended cold region rather than
concentrated in the nuclear regions. We discuss the implications of this study
for understanding populations detected at other wavelengths, such as the bright
850um SCUBA sources or the faint Spitzer 24um sources.
