Nuclear power is generated either through fusion or through fission.
Fission is the splitting of a heavy nucleus into light nuclei that are more energetically favorable. Since Iron-56 is the most energetically favorable nucleus, both the fission of heavy nuclei and the fusion of light nuclei can release energy. Fission is the process used in the first nuclear weapons and in power plants. One drawback to its use as a power source is that among the heavy nuclear fragments it produces are radioactive elements with long half-lives. Fission is set off by bringing together a critical mass of an element such as Uranium-235.