A supernova is the giant explosion of a star. There are two main classes: core collapse (SNe II and SNe Ib/c) and thermonuclear (SNe Ia).
The core collapse SNe are massive stars at the end of their nuclear burning lives. Their cores collapse to neutron stars or black holes and release neutrinos. The neutrinos captured in the layers above the proto-neutron star are the essential cause of the explosion.
Thermonuclear SNe are believed to be the explosions of white dwarfs: mass accretion from a companion builds the WD up to nearly the Chandrasekhar mass limit, but before collapse the WD explodes.
Both SN classes produce expanding remnants that interact with the interstellar medium (ISM) and inject heavy elements into the ISM.
Sites
Individual Supernovae