There is no difference. The word fission simply means splitting. Nuclear energy can be obtained in two ways: fission and fusion. Fission is a nuclear process in which a heavier unstable nucleus divides or splits into two or more lighter nuclei, with the release of substantial amounts of energy. When a free neutron of the proper energy is captured by the nucleus of a fissionable atom, the resulting unstable nucleus will split producing two or more fission products (atomic nuclei of different elements formed from the protons and neutrons originally comprising the nucleus before its fission), two or three free neutrons and a tremendous amount of energy. Nuclear fusion (a thermonuclear reaction) is a process in which two nuclei join to form a larger nucleus, thereby giving off energy. Nuclear fusion is the energy source which causes stars to "shine", and hydrogen bombs to explode. Any two nuclei can be forced to fuse with enough energy. When lighter nuclei fuse, the resulting nucleon has too many neutrons to be stable, and the neutron is ejected with high energy. Most lighter nuclei will produce more energy than initially required to cause them to fuse, making the reaction exothermic and chain or transiently self-sustaining, and generating net power. For the opposite case, heavy nuclei with too few neutrons are also unstable and lead to nuclear fission. Unlike fusion however, fission reactions require so little extra energy for very heavy nuclei that they occur spontaneously, all the time. This is not the case with fusion, where the lowest mass nucleon, hydrogen, still requires considerable energy to fuse.