The path of the raction occurs on the basis of mass of the nuclei involved in reaction.
In case of nuclear fusion, two or more nuclei having less mass fuse(combine, join) together to form a new nuclei(heavier mass but it is relatively stable). During fusion, matter is not conserved because some of the matter is converted into energy(light). This reaction evolves a huge amount of energy and there comes Einstein's famous Energy-mass equivalence formula E=mc^2! :D. The nuclear reaction occuring in stars(including our sun ) is "fusion".
Fission occurs with heavier nuclei such as that of Uranium-235. Which splits into smaller subatomic particles like gamma, neutrons and enormous amount of energy.
Both, Fission and Fusion releases enormous amount of energy and modern nuclear weapons works on the principle of nuclear fission.
