Just in case someone skipped out on the relevant day of middle school science, it should be 'fission'.
The difference is that fission is taking heavy elements, breaking them apart and using the released energy. Radioactivity, Thorium, Uranium, Plutonium, all the yucky stuff. Research has been going on for decades into making these reactors safer, and with breeder reactors and modern conventional designs that appears to have been achieved. Nevertheless, they appear on their way out anyway.
Fusion fuses together light isotopes and uses the energy thus released - they basically do what the sun does. And are clean. For various definitions of clean.
And if you're wondering how they are the complete opposite yet both work, it all pivots around iron. Copying from Wikipedia on iron:
> Its abundance in rocky planets like Earth is due to its abundant production by fusion in high-mass stars, where the production of nickel-56 (which decays to the most common isotope of iron) is the last nuclear fusion reaction that is exothermic.
And yes, that means that everything in this universe will one gigayear end up right at the pivot between fission and fusion, ever onwards oscillating further towards it. Our universe is ever tending tiwards irony. A pivot that we're probably going to see humanity go through too, but in a different way. Hopefully sooner rather than later.
