As part of the attempt to understand supernova explosions, astronomers have classified them according to the lines of different chemical elements that appear in their spectra. See "Optical Spectra of Supernovae" by Filippenko ( Annual Review of Astronomy and Astrophysics, Volume 35, 1997, pp. 309-355 (http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1997ARA%26A..35..309F&db_key=AST&high=3f6510b0d828671)) for a good description of the classes.
The first element for division is the presence or absence of a line from hydrogen. If a supernova's spectrum contains a hydrogen line, it is classified Type II, otherwise it is Type I.
Among those groups, there are subdivisions according to the presence of other lines and the shape of its light curve.