Nuclear fission and fusion are two alternative ways to generate electrical energy. Although the names sound similar they are in fact opposites of each other.
Nuclear fission is splitting atoms to create energy in the form of heat and radiation which can then be used to create electrical energy. The main element used in nuclear fission is Uranium. This process creates a large amount of energy but the drawbacks are that it can be very dangerous (fission can create atomic bombs) and produces radioactive waste which is difficult to dispose of.
Nuclear fusion is combining the nuclei of light elements together to create energy. However, this form of creating energy is less effective at the moment because scientists haven’t quite yet figured out how to perfect their methods to create electrical energy from the process.