A supernova is actually an event rather than a specific thing. Supernova is the name given to the explosion of a large star (mass of 1.5 times that of our Sun or larger) and its aftermath. A supernova remnant is the energy and matter left after the explosion.
The life span of any star is determined by the amount of matter available to burn - the cause of the light energy we can see. As the last of the fuel is exhausted, the star begins to collapse in on itself, emitting tremendous amounts of energy. As the core becomes more concentrated and hotter, outer layers of matter particles expand. Eventually, the remains of the star explode with one final outburst of radiation and matter, forming a supernova that we may observe in a shell-like appearance (example: the Cygnus Loop) or in an irregular blob (example: the Crab Nebula).
All the elements heavier than iron were made in a supernova explosion, so the only reason we find these elements on Earth or in our Solar System — or any other extrasolar planetary system — is because those elements were formed during a supernova.