  Supernovae (SNe), the luminous explosions of stars, were observed since
antiquity, with typical peak luminosity not exceeding 1.2x10^{43} erg/s
(absolute magnitude >-19.5 mag). It is only in the last dozen years that
numerous examples of SNe that are substantially super-luminous (>7x10^{43}
erg/s; <-21 mag absolute) were well-documented. Reviewing the accumulated
evidence, we define three broad classes of super-luminous SN events (SLSNe).
Hydrogen-rich events (SLSN-II) radiate photons diffusing out from thick
hydrogen layers where they have been deposited by strong shocks, and often show
signs of interaction with circumstellar material. SLSN-R, a rare class of
hydrogen-poor events, are powered by very large amounts of radioactive 56Ni and
arguably result from explosions of very massive stars due to the pair
instability. A third, distinct group of hydrogen-poor events emits photons from
rapidly-expanding hydrogen-poor material distributed over large radii, and are
not powered by radioactivity (SLSN-I). These may be the hydrogen-poor analogs
of SLSN-II.
