Nuclear fission and nuclear fusion both involve radioactivity, both yield enormous amounts of energy, and both are used in nuclear weapons. So, how can you tell them apart?
Fission reactions involve breaking large atomic nuclei into smaller pieces. For example, uranium can be fissioned into strontium and krypton.
Fusion reactions involve fusing or joining atomic nuclei together. The products of fusion will contain more protons or neutrons than the reactants. For example, helium (atomic number of 2) is produced in the sun from hydrogen (atomic number of 1) through nuclear fusion.