Nuclear power extracts energy by rearranging the cores of atoms. Fission splits one heavy nucleus into pieces. Fusion forces two light nuclei into one. Both release energy because the products weigh slightly less than what went in — and that lost mass becomes energy.
Every nuclear power plant operating today runs on fission. A slow-moving neutron strikes a uranium-235 nucleus, which becomes unstable and splits — flinging out fragments, energy, and 2–3 fresh neutrons that go on to split more atoms.
Fusion powers the sun and remains the great engineering challenge. Two hydrogen isotopes — deuterium and tritium — must be heated past 100 million °C so their nuclei move fast enough to overcome their mutual repulsion and merge into helium. Drag the temperature up and watch.
Same underlying principle — convert a sliver of mass into energy — pursued in opposite directions.
The one-line summary: fission is mature but dirty, splitting heavy atoms in reactors running right now. Fusion is cleaner and denser but unsolved — we can't yet keep a sun-hot plasma contained long enough to get more energy out than we put in.