What is Nuclear Fission?

Nuclear energy is the energy trapped inside of an atom. Nuclear fission is when an atom's nucleus is split apart releasing an tremendous amount of energy. The energy released is both light and heat energy.
When a nucleus of an atom fissions it splits into several smaller fragments, usually two different fragments or fission products. The fission products are equal or half the mass of the original mass of the atom. During the process of fission two or three neutrons are emitted, large quantities of energy is released and radioactive products are formed.
Fission can occur spontaneously or when a nucleus of a heavy atom captures a neutron.
Fissile isotopes are isotopes of an element that can be split through fission. There are only certain isotopes that are fissile (able to fission). An important factor that will determine whether or not isotopes will fission, is the speed of the bombarding neutron. Some isotopes such as thorium-232 requires a very fast moving neutron for fission, where as uranium-235 needs a slow moving neutron to fission. If a neutron is too fast it will pass right through the atom which will not affect the isotope at all.