The explosion of a star. Supernovae come in two types: Type I is caused by sudden nuclear burning in a white dwarf star. Type II is caused by the collapse of the core of a supermassive star at the end of its nuclear-burning life. In either case, the star is destroyed and the light given off in its explosion briefly rivals the total light given off by a whole galaxy.
A Supernova Remnant is the material blown off during a supernova, now seen as a great glowing cloud expanding into space.
Figure: A supernova remnant. This is the Crab Nebula, the leftover ejecta from a supernova explosion that took place in 1054 and was recorded in the records of Chinese astronomers. The remnant is kept glowing by energy from the rotating neutron star ("pulsar") at its center.