How does nuclear fission differ from nuclear fusion? Why are both of these processes exothermic?

A nuclear reaction in which heavy nucleus of an atom splits into two smaller nuclei along with release of energy is known as nuclear fission.

Whereas a nuclear fusion reaction is the one in which two small nuclei combine together and results in the formation of a larger nuclei along with release of energy.

In both these nuclear reactions, energy is released and thus, they are exothermic in nature. This is because in nuclear fission, energy absorbed for splitting of atom is less than the amount of energy released. As a result, nuclear fission is also exothermic in nature.

And, in nuclear fusion small nuclei combines that leads to the formation of new nuclei. Hence, always energy is released in this reaction.