Answer:
Fission reaction is the splitting of heavy nucleus, by bombardment with a neutron, into mainly two fragments of smaller nuclei plus 2-3 neutrons and release of energy. Example is the nuclear fission of Uranium-235.
Fusion reaction is the merging of two or more light nuclei upon collision and formation of a heavier nucleus and release of energy. Example is merging of four hydrogen nuclei into one helium nucleus and release of energy. This is the source of solar energy.