Nuclear fusion is a nuclear process in which two or more light nuclei combine or fuse to form a heavier nucleus with the release of a large amount of energy.

To bring the two lights nuclei together in a fusion process, a very high temperature of the order 10,600°C is required t0 overcome the Coulomb repulsive forces between the two nuclei.

Nuclear fission is the splitting up of the nucleus of a heavy element into two approximate equal parts with the release of a huge amount of energy and neutrons

Fission occurs with most of the massive nuclei. When the heavy nucleus is bombarded by a slow neutron, several neutrons are produced as      by-products. These neutrons may cause the splitting of other nuclei, which in turn yield more neutrons which may further split other nuclei and so on.
Thus chain reaction is set in motion.

A chain reaction is a multiplying and self-maintaining reaction. When the size of the nuclei exceeds a certain critical mass, there is a rapid production of neutron accompanied by a release of a tremendous amount of energy in a nuclear explosion. This is the principle of the
atomic and nuclear fission bombs. It is also the process used in the present-day nuclear power station.