Fission is the splitting of heavy nuclei (such as uranium) – in two smaller nuclei. This process needs less energy to ‘bind’ them together – so energy is released. Fission happens quite easily – and is used to generate electricity in conventional nuclear power stations.
Fusion on the other hand, is the process of sticking together light nuclei (typically hydrogen-like nuclei). The larger nuclei again needs less energy to hold it together – so energy is released. This is what happens in the Sun and stars, and research on how to harness fusion energy on Earth is being carried out in devices such as tokamaks and stellarators.
While it might seem confusing that energy can be generated by both fusion and fission , as they appear to be quite opposite processes, the explanation lies in the size of the nuclei.
Light elements, such as hydrogen and helium, have small nuclei that release lots of energy when they fuse together. Moving to heavier atoms, less energy is released in each fusion event; until, at iron (26 protons and 30 neutrons), no more energy is released by fusion. Any bigger, it takes energy to make fusion happen. Atoms with really huge nuclei, such as uranium and plutonium do the opposite of fusion: they release energy when they break apart.