What is the difference between nuclear fission and fusion?

Nuclear fission is the process that splits an atom nucleus apart and release energy.

Normally a neutron is struck against an atomic nuclei and splits it into two smaller nuclei, releasing two or three neutrons that go on to split even more nuclei. Those nuclei release even more neutrons. This is called a chain reaction. Think of it like the dominos effect. This can be conducted on Earth in what we called nuclear reactors and we harness and use the energy released.

Nuclear fusion is the process that joins two atomic nuclei together to create a bigger one, whilst also releasing energy.

This happens when two atoms impact each other at an enormous speed. This is how stars release energy. However no current invention allows nuclear fusion to be conducted on Earth.