nik's right... the hit, their nuclei fuse (hence "fusion") and a lot of materials that can't fit get launched out in the form of energy and sub-atomic particles. Said particles smack into other atoms, possibly causing a fission breakage. The larger of these blocks created from fission can colide together and fuse back into atoms, releasing more energy. That's why it's a chain reaction. Granted, eventually, if you have a perfect reaction, you'll burn up all the spare matter and turn it all into energy.
The sun uses hydrogen and helium. We can't generate a sustained reaction using those lightest of elements, so we use unstable elements (uranium and plutonium) which only sustains until the heavy matter breaks down into lead and radon and radioactive alpha-particles and stuff.
