V838 Mon has undergone one of the most mysterious stellar outbursts on record. The spectrum at maximum closely resembled a cool AGB star, evolving toward cooler temperatures with time, never reaching optically thin conditions or showing increasing ionization and a nebular stage. The latest spectral type recorded is M8-9. The amplitude peaked at ΔV=9 mag, with the outburst evolution being characterized by a fast rise, three maxima over four months, and a fast decay (possibly driven by dust condensation). BaII, LiI and s-element lines were prominent in the outburst spectra. Strong and wide (500 km/sec) P-Cyg profiles affected low ionization species, while Balmer lines emerged to modest emission only during the central phase of the outburst. A light-echo discovered expanding around the object constrains its distance to 790+/-30 pc, providing MV = +4.45 in quiescence and MV = -4.35 at optical maximum (dependent on the still uncertain EB-V=0.5 reddening). The visible progenitor resembles a somewhat under-luminous F0 main sequence star, that did not show detectable variability over the last half century. V838 Mon together with M31-RedVar and V4332 Sgr seems to define a new class of astronomical objects, Stars that Erupt into Cool Supergiants (SECS). They do not develop optically thin or nebular phases, and deep P-Cyg profiles denounce large mass loss at least in the early outburst phases. Their progenitors are photometrically located close to the Main Sequence, away from the post-AGB region. After the outburst, the remnants still closely resemble the precursors (same brightness, same spectral type). Many more similar objects could be buried among poorly studied variable stars that have been classified as Miras or SemiRegulars on the base of a single spectrum at maximum brightness.