What is the difference between nuclear fusion and nuclear fission?

Both the processes of nuclear fusion and fission are dictated by the concept of a 'binding energy'. In basic terms, this is the energy required to separate an atomic nucleus into is constituent nucleons, i.e. its protons and neutrons. In nuclear reactions, if a nucleus is converted into a new system of nuclei with higher binding energy, the excess binding energy is released in the form of energetic particles and radiation. (Show a graph of binding energy vs mass number)
Nuclear fission involves the splitting of a nucleus with a high mass number into two lighter nuclei. Energy is released during this process as a result of the fact that the sum of the binding energies of the two lighter nuclei is greater than the parent nuclei's binding energy. On the other hand, nuclear fusion involves the combining of two lighter nuclei to form a heavier nucleus. Here the heavier nucleus has a greater binding energy than the two parent nuclei combined. Overall, the process of fusion releases more energy than fission but currently only fission is used in commercial nuclear power plants.