Nuclear Fusion is the process of bringing two or more atoms together in a way that the atoms fuse, casting off excess electrons in the form of radiation. Fusion has an upper weight limit beyond which the atoms will not fuse on their own and require external energy; this is the Iron Limit, at the isotope Iron-56.
Current nuclear power reactors use Fission reactions, using either self-sustaining or non-self-sustaining fissions in radioactive material. By bringing radioactive material closer or farther from the reactant, we are able to control the output of power; however, the material eventually decays and must be replaced.
In a nuclear fusion power reactor, sustained reactions would come from the continuous fusing of atoms below the iron limit. Since the resulting output of energy is extremely hot, there are problems with controlling and containing the reactant material. Magnetic containment has by itself been unsuccessful, with leaks and uneaven heating. At the moment, no commercial fusion power source has been successfully completed.
The Tokamak, a plasma field containment unit shaped like a donut, had been the most successful model to date. There have been many built over the last forty years and they are likely to be the eventual form of the sustainable nuclear fusion reactor in the future.
Cold fusion has never been successfully demonstrated and can be considered only a theory at this point.
The failure to create a sustainable fusion reactor leads one to believe that continuing research will be futile, and yet there have been demonstrated successes using small, non-commercial reactors. By continuing to experiment with different methods of containing and utilizing hot fusion reactions, we can build safer containment units for existing fission reactors, as well as building safe storage for nuclear waste. These experiments also increase our understanding of magnetic properties.