What goes on inside a massive star before it explodes as a supernova?

When most people think of a supernova, they're thinking of a Type II core-collapse supernova. These are massive stars that have reached the end of their time on the main sequence. They've used up their supply of hydrogen and continue fusing heavier elements until the star can't support its own mass. The core collapses and they explode, outshining their entire host galaxy for months.