I will elaborate however.
Fission and Fusion are reactions that alter the very structure of mater; they are not mere re-arrangements like relationships on a soap opera, but entirely new things such as trading in two bicycles for a motorcycle.
Fusion (hydrogen) is something most recently Hollywood physics bent to make Iron Man's arc reactors; the (SPOILER movie #2) adding what is effectively a reaction catalyst/medium. The giant doughnut reactor you see in the first movie is a good 'inner' view of what one of these may actually look like if built in a working way.
Fission is the easier and more realistic matter splitting type.
In both cases the general idea is that shifting the configuration of mater results in a few different pieces of mater, some 'leftover parts' that go on to further the reaction (alpha, beta, gamma; radiation) and a /lot/ of heat which can be used in more conventional energy production methods, such as steam turbines.
The Fission reactions all tend to be 'messy' because they leave a bunch of unstable leftovers laying about that can take quite a long time to stabilize. This is /partly/ due to the fact that fuel reprocessing to concentrate it back to reactor grade material is banned in the US due to a retarded fear that all such activity /could/ lead to weapons proliferation (as if dirty bombs aren't nearly as bad). Were that material reprocessed and the full energy extracted the waste products would largely fall in to two categories: things with very short half-lives (go away in a few days to weeks at most) and things with -very- long halflives (a slow trickle of radiation; extremely useful in medical applications and nuclear based batteries; possible applications in electronics).
Fusion reactions, meanwhile, are notorious for having issues actually producing a net gain of energy. I hope we'll be able to build such systems eventually, but within the short term gen III/IV fission reactors and reprocessing are the options I'd love to see in use. Remember, it wasn't the earthquake it's self that ravaged the older, poorly run and management crippled reactors in Fukushima; it was poor management and failure to account for tsunami flooding with properly laid out equipment that produced that result. Newer plants would doubtlessly be built even more idiot proof (with automated systems to do what was necessary in the first place) and with more extreme murphy's law scenario writers listened to.