Nuclear fission (in nuclear physics, simply fission) is a process in which the nucleus of an atom splits into two or more smaller nuclei (fission products) and usually some by-product particles.
Hence, fission is a form of elemental transmutation. The by-products include free neutrons, photons (usually gamma rays), and other nuclear fragments such as beta particles and alpha particles. Fission of heavy elements can release substantial amounts of useful energy both as gamma rays and as kinetic energy of the fragments.
Nuclear fission is used to produce energy for nuclear power and to drive explosion of nuclear weapons. Fission is useful as a power source because some materials, called nuclear fuels, both generate neutrons as part of the fission process and also undergo triggered fission when impacted by a free neutron. Nuclear fuels can be part of a self-sustaining chain reaction that releases energy at a controlled rate (in a nuclear reactor) or a very rapid uncontrolled rate (in a nuclear weapon).