Fission and fusion are different ways that nuclear energy can be produced.
Fission is the process of splitting apart the nucleus of an atom. When the nucleus of an atom is split apart, energy is released. Both heat and light energy are produced by fission.
A nuclear power plant splits apart uranium atoms in a chain reaction in order to produce energy. The splitting of uranium atoms in a nuclear power plant is controlled in order to ensure that the reaction doesn't move too fast.
Rods of fuel containing uranium atoms produce heat energy that is used to boil water. Water that is heated by the core is used to produce steam that turns a turbine. The turbine is used to generate electricity.
Fusion is the process of joining smaller nuclei in order to form a larger nucleus. The sun is able to combine atoms of hydrogen in order to produce helium atoms. The process of fusion is how atoms are formed. When atoms are joined in fusion energy is produced.
The scientific community has been working to harness and control fusion reactions to create a reliable energy source. Fusion reactions produce less harmful waste than fission reactions and the fuel source for a fusion reaction could last much longer.