What is a supernova?
A stars life is measured in segments, our star (the sun) is about 5 billion years old, and has another 5 billion years or so. Our sun is said to be in its main sequence, it is hot enough to turn hydrogen into helium, which creates a great deal of energy, the same energy we use to get a tan. Eventually the sun will run out of hydrogen, at which point it will become very large and red, it will then be known as a red giant.
This red giant has run out of hydrogen but it keeps on burning the helium until is turns to carbon, then oxygen, neon, silicon, sulfur, and finally to iron. This burning process adds protons to the elements changing them to a heavier element, ie hydrogen has only one proton in it, helium has two, iron has 26 protons (this is how an atomic number is measured as well). Iron is very stable and cannot be burned to a heavier element so the sun, which was burning itself into oblivion, cannot contract further, the energy that would normally fall inward is bounced out by the an iron core. This bounce produces a supernova explosion. This explosion lasts a full month and is brighter than a while galaxy of stars.
Not all stars will supernova however, ours will not. Only massive stars will supernova. Ours will die more gracefully, once it runs out of hydrogen it will turn into a red giant, and consume the Earth. When it runs out of helium will be big enough to reach Jupiter and after a few tens of thousands of years it will lose its mass and shirk to a hot core of carbon and our solar system will turn into a nebula. This carbon core will cool and turn into a white dwarf, providing little light and heat.