So what's nuclear fusion? Basically, the reverse of fission, which powers current nuclear plants (and A-bombs).

Imagine you're really tiny, tiny enough to look at an atom and see its constituents. It'll have a nucleus at the center, and electrons flying around it. The nucleus itself has two types of elements, neutrons and protons. If you were to weigh these protons and neutrons with a scale, they'd actually weigh more than the nucleus itself. The difference is the energy that binds these protons and neutrons together (remember e=mc²? energy and mass are the same thing). This binding energy can be released by playing with the weight of the nucleus. Hence nuclear reactions.

Atoms all weigh different and binding energy varies from atom to atom, but basically the atoms in the middle of the weight scale have low binding energy, and the really light ones (like hydrogen) or heavy ones (like Uranium and Plutonium) have much higher binding energy. So if we can split a very heavy atom into parts that are more stable, we've just released energy. That's fission, and that's what powers all nuclear plants around the world. Conversely, we can also fuse very light atoms together and also release energy. That's fusion, which powers the biggest nuclear bombs, the H bombs.

The great thing about fusion is that it produces very little waste, and the little waste it produces ceases to be radioactive after a few decades (as opposed to hundreds of thousands of years for Plutonium). In the best case scenario, we could take hydrogen isotopes (atoms that are "cousins" of hydrogen) commonly found in sea water, fuse them together, and release a boat load of energy. Now, this is extremely complex stuff, much more so than fission. Why? Because a heavy atom can be split by throwing a neutron at it, that's easy. But nucleii don't like each other, at all, so to fuse them together, you have to basically throw them at each other really fast, meaning at really high temperatures (hotter than the surface of the sun). The fuse for an H-bomb is actually an A-bomb — it takes that much energy and heat to start a nuclear fusion reaction.

Hence the need for a really big machine to try to do that.

Scientists will probably succeed within the next 50-75 years. At that point, we'll basically be able to power our houses with sea water. And in some configurations, it might be possible to even burn our current radioactive waste in specialized power plants. The possibilities are mind-boggling, it truly is the biggest challenge to science since people discovered electricity.

So, Go ITER!