Nuclear energy from fission and fusion
Nuclear power is generated either through fusion or through fission. Fission is the splitting of a heavy nucleus into light nuclei which 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. Fusion is the source of the sun's energy and is the source of energy in hydrogen bombs. Fusion is an inherently cleaner source of energy, but igniting it in a controlled way has proved problematic. (hydrogen bombs are set off by fission devices).
Fission is set off by bringing together a critical mass of an element such as Uranium-235. If a sufficient amount of Uranium is brought together an emitted neutron will most likely be captured, changing the Uranium to U-236, rather than escaping from the surface. Since U-236 decays and produces 2 neutrons, the number of neutrons grows exponentially, and practically all the U-235 nuclei decay in short order. This is called a chain reaction. In a power plant the chain reaction is modified with neutron absorbing rods.