  We present a multi-wavelength catalog (15 um, R, K-band, 1.4 GHz flux) plus
spectroscopic identifications for 406 15 um sources detected in the ELAIS
region S1, over the flux density range 0.5<S<150 mJy. 332 (~82%) sources are
optically identified down to R~23.0. Spectra or bona fide stellar
identifications are obtained for 290 objects. The areal coverage, MIR and
optical completeness of the sample are discussed in order to allow statistical
and evolutionary analyses. Two main spectroscopic classes have been found to
dominate the MIR extragalactic population: z<0.5 star-forming galaxies (from
absorbed to extreme starbursts: nuL_nu~10^8-10^11 L_odot), which account for
\~75% of the sources, and AGN (both type 1 and 2), which account for ~25% of
the sources. About 20% of the extragalactic sources are dust-enshrouded
starburst galaxies [e(a) spectra], and all the starburst galaxies appear more
dust extincted in the optical than nearby normal galaxies. We also identified
91 stellar objects (~22% of the MIR sources). The counts for starburst galaxies
and AGN down to 0.6 mJy have been derived. A general trend is found in the
optical-MIR SED of the galaxies, where the MIR-luminous objects have larger MIR
to optical luminosity ratios.
