When hydrogen fuses with hydrogen you get helium and a neutron or proton (depending on what hydrogen isotopes were fused) This weighs less than the two original hydrogen isotopes. The mass lost is converted to energy due to mass–energy equivalence (that E=mc2 formula)
I see, makes sense. Small amount of mass converts into a high amount of energy. Making this viable would be a major step in getting rid of fossil fuels and nuclear power. And making energy production safer too, the recent oil spill proves you don't need an explosion for a disaster.