Nuclear fission is the splitting of a heavy nucleus to form two nuclei
of roughly the same mass, in addition to several neutrons. Fission
rarely occurs spontaneously and is induced when a single neutron
collides with a nucleus being captured as a result. This releases
neutrons, which in turn may cause further fission and so on in a chain
reaction. This process releases a sizeable amount of energy such that
the energy released per atom by fission is roughly 50 million times
greater than that per atom from a chemical reaction such as burning.
Uncontrolled chain reactions are used in nuclear weapons while controlled ones take place in nuclear reactors and realease at a steady rate. To maintain a chain reaction, a minimum of one neutron from each fission must cause further fission. If the fission material is less than a certain critical size, too many neutrons escape without hitting nuclei.
Picture Source - http://www.needham.mec.edu/NPS_Web_docs/High_School/cur/mp/photos/fission.gif