deary me people do i need to explain nuclear physics to you again? come on class it's very simople.

fission is when you fire a neutron at a rather large, dense and unstable atom. this breaks it into smaller bits. and neutrons. the neutrons then go on to hit other atoms and so the process continues. this is called an uncontrolled chain reactiion. how does it produce energy? well energy = mass multiplied the speed of light squared. (e=mc2) mass is lost during a chain reaction somewhere. and the loss mass becomes energy. not alot per atom really. but there area few billion atoms in any given lump of stuff. so alot of enery is produced.

fusion is when two aroms hit together and join together. like two hydrogen isotopes (just read isotope as atom if you don't know what an isotope is) when the get bashed together they can join and become a helium isotope. and again the combined weight of the two hydrogen is more than the product helium so the mass is given off as energy. it's a cleaner and more effeceint method of generating energy than fusion as it doesn't produce all sorts of nasty radioactive waste. also salt water is far easier to come by than plutonium and uranium. and cheaper to get out too.