  Combining the relative vicinity of the Local Group spiral galaxy M33 with the
Spitzer images, we investigate the properties of infrared (IR) emission sites
and assess the reliability of the IR emission as a star formation tracer. The
mid- and far-IR emission of M33 was obtained from IRAC and MIPS images from the
Spitzer archive. We compared the photometric results for several samples of
three known types of discrete sources (HII regions, supernovae remnants and
planetary nebulae) with theoretical diagnostic diagrams, and derived the
spectral energy distribution (from 3.6 to 24 micron) of each type of object.
Moreover, we generated a catalogue of 24 micron sources and inferred their
nature from the observed and theoretical colours of the known type sources. We
estimated the star formation rate in M33 both globally and locally, from the IR
emission and from the Halpha emission line. The colours of the typical IR
emissions of HII regions, supernovae remnants and planetary nebulae are
continuous among the different samples, with overlapping regions in the
diagnostic diagrams. The comparison between the model results and the colours
of HII regions indicates a dusty envelope at relatively high temperatures ~600
K, and moderate extinction Av < 10. The 24 micron sources IR colours follow the
regions observationally defined by the three classes of known objects but the
majority of them represent HII regions. The derived total IR luminosity
function is in fact very similar to the HII luminosity function observed in the
Milky Way and in other late type spirals. Even though our completeness limit is
5x10^37 ergs s-1, in low density regions we are able to detect sources five
times fainter than this, corresponding to the faintest possible HII region.
[abridged]
