  We cross-matched Wide-field Infrared Survey Explorer (WISE) sources brighter
than 1 mJy at 12um with the Sloan Digital Sky Survey (SDSS) galaxy
spectroscopic catalog to produce a sample of ~10^5 galaxies at <z>=0.08, the
largest of its kind. This sample is dominated (70%) by star-forming (SF)
galaxies from the blue sequence, with total IR luminosities in the range
~10^8-10^12 L_sun. We identify which stellar populations are responsible for
most of the 12um emission. We find that most (~80%) of the 12um emission in SF
galaxies is produced by stellar populations younger than 0.6 Gyr. In contrast,
the 12um emission in weak AGN (L[OIII]<10^7 L_sun) is produced by older stars,
with ages of ~1-3 Gyr. We find that L_[12um] linearly correlates with stellar
mass for SF galaxies. At fixed 12um luminosity, weak AGN deviate toward higher
masses since they tend to be hosted by massive, early-type galaxies with older
stellar populations. Star-forming galaxies and weak AGN follow different
L_[12um]-SFR (star formation rate) relations, with weak AGN showing excess 12um
emission at low SFR (~0.02-1 M_sun/yr). This is likely due to dust grains
heated by older stars. While the specific star formation rate (SSFR) of SF
galaxies is nearly constant, the SSFR of weak AGN decreases by ~3 orders of
magnitude, reflecting the very different star formation efficiencies between SF
galaxies and massive, early-type galaxies. Stronger type II AGN in our sample
(L_[OIII]>10^7 L_sun), act as an extension of massive SF galaxies, connecting
the SF and weak AGN sequences. This suggests a picture where galaxies form
stars normally until an AGN (possibly after a starburst episode) starts to
gradually quench the SF activity. We also find that 4.6-12um color is a useful
first-order indicator of SF activity in a galaxy when no other data are
available.
