  This is the first paper in a series devoted to review the main properties of
galaxies designated S0 in the Hubble classification system. Our aim is to
gather abundant and, above all, robust information on the most relevant
physical parameters of this poorly-understood morphological type and their
possible dependence on the environment that could later be used to assess their
possible formation channel(s). The adopted approach combines the
characterisation of the fundamental features of the optical spectra of
$68{,}043$ S0 with heliocentric $z\lesssim 0.1$ with the exploration of a
comprehensive set of their global attributes. A principal component analysis is
used to reduce the huge number of dimensions of the spectral data to a
low-dimensional space facilitating a bias-free machine-learning-based
classification of the galaxies. This procedure has revealed that objects
bearing the S0 designation consist, despite their similar morphology, of two
separate sub-populations with statistically inconsistent physical properties.
Compared to the absorption-dominated S0, those with significant nebular
emission are, on average, somewhat less massive, more luminous with less
concentrated light profiles, have a younger, bluer and metal-poorer stellar
component, and avoid high-galaxy-density regions. Noteworthy is the fact that
the majority of members of this latter class, which accounts for at least a
quarter of the local S0 population, show star formation rates and spectral
characteristics entirely similar to those seen in late spirals. Our findings
suggest that star-forming S0 might be less rare than hitherto believed and
raise the interesting possibility of identifying them with plausible
progenitors of their quiescent counterparts.
