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This is the first paper in a series devoted to review the main properties ofgalaxies designated S0 in the Hubble classification system. Our aim is togather abundant and, above all, robust information on the most relevantphysical parameters of this poorly-understood morphological type and theirpossible dependence on the environment that could later be used to assess theirpossible formation channel(s). The adopted approach combines thecharacterisation of the fundamental features of the optical spectra of$68{,}043$ S0 with heliocentric $z\lesssim 0.1$ with the exploration of acomprehensive set of their global attributes. A principal component analysis isused to reduce the huge number of dimensions of the spectral data to alow-dimensional space facilitating a bias-free machine-learning-basedclassification of the galaxies. This procedure has revealed that objectsbearing the S0 designation consist, despite their similar morphology, of twoseparate sub-populations with statistically inconsistent physical properties.Compared to the absorption-dominated S0, those with significant nebularemission are, on average, somewhat less massive, more luminous with lessconcentrated light profiles, have a younger, bluer and metal-poorer stellarcomponent, and avoid high-galaxy-density regions. Noteworthy is the fact thatthe majority of members of this latter class, which accounts for at least aquarter of the local S0 population, show star formation rates and spectralcharacteristics entirely similar to those seen in late spirals. Our findingssuggest that star-forming S0 might be less rare than hitherto believed andraise the interesting possibility of identifying them with plausibleprogenitors of their quiescent counterparts.

The local universe in the era of large surveys. I. Spectral classification of S0 galaxies