Today I'm going to post one of my favorite graphs. It's a simple plot of the binding energy of different elements against the size of those elements' nuclei, but its implications are deep enough to drive furious research and millions of dollars in spending. Wikipedia explains the basic idea of binding energy nicely - it's the energy required to take apart a nucleus into its individual protons and neutrons. When you go the other way, putting nucleons together to make a nucleus, you increase the binding energy and release kinetic energy that can be used by us in some form or another (many nuclear power plants, for example, use this energy to generate steam to run turbines to generate electricity).
Getting to the point, here's the graph. When you grok how it applies to getting energy from fusion and fission, you will say, "Woah."
Fusion and fission are processes that change the number of nucleons (atomic number, A) of an atom. The energy they release is equal to the difference in binding energy between the before element and the after element. Fusion combines small nuclei to make bigger ones, so fusion takes you from left to right on the graph. Fission breaks apart large nuclei into smaller ones, so it takes you from right to left on the graph. Iron (Fe) is at the top - both processes stop there.
Nuclear power plants work by fission, from right to left. Look at the difference between Uranium 235 and the next few elements down - not much of a difference in binding energy. Not much energy is released to be used.
Now look at the difference in binding energy between hydrogen (H) and helium (He). Yeah, pretty impressive! I love this graph because it so elegantly and subtlely tells you exactly why power from nuclear fusion is such a holy grail.