Fusion - Thanks to NASA
Nuclear physics is the process of getting energy from the nucleus of an atom. There are two ways to release energy from atoms: fission and fusion. Fusion happens all the time in nature, mostly inside stars because fusion needs lots of heat or gravity to get it going. Fusion works with the smallest atoms. Four hydrogen atoms smash together to make one helium atom, and they release the energy you see as sunshine and starshine. People can also cause fusion reactions inside hydrogen bombs.
Fission - an atomic bomb
Fission doesn't happen much in nature, but people can use fission reactions to release energy for atomic bombs, and also for nuclear power plants to make electricity for houses and cars and factories. Fission works with the biggest atoms like uranium. Splitting these big atoms apart releases energy too.