An atom of Uranium is large - well by atomic standard it is, 235 or more protons and neutrons and it only just holds together. Give it a shove with a few stray neturons and it'll split.
When it splits it releases energy this is fision - it's the way nuclear power stations work.
Hydrogen is the smallest element - 1 proton maybe a neutron or two. If you can get a couple of them close enough they'll stick together and make Helium and there will be energy left over. This is fusion and is how the sun shines.
If we could make a power station work on fusion rather than fission there would be big advantages - the raw fuel would be water, the safety aspects would be vastly better than todays power stations and the waste would be tiny and short lived.
But it's difficult - we've been at if for 50 years and it'll probably be about another 40 before you boil your kettle from one.
It's hard because you have to get those hydrogens close enough and the electric charges push them away like trying to put 2 magnets together so they have to be hot , very hot as hot as the sun.
About 20 years ago it was claimed that people had found a way to do it at room temperature (cold fusion)- I was working as a physicist on fusion experiments at the time and the science was pretty nonsense - we rather groaned as it was clearly going to be a media sideshow -it was.
The US military still fund a bit of research on it as a kind of lottery ticket but don't hold your breath.
Meanwhile ITER is being built in France
http://www.iter.org/ which will be followed by DEMO - the first fusion power station
It's been a long time coming but the rewards are enormous.