In a fusion reactor unstable elements, in plasma form that are positively charged are forced together. The reactor can be up to 1,000,000,000 degrees Celcius. The end result is larger nuclei and large amounts of energy. The reason that it gives off so much energy is because in Einstein's equation for his theory of energy is equal to mass times the speed of light squared. The amount of mass in the reactor multiplied by the speed of light squared will show you how much energy will be produced.
Fusion is what causes the sun to produce energy.
The hydrogen fuses with other elements on the sun. The high tempetature and pressure on the sun are perfect for the fusion process to take place. To the Main Chemistry Page To the Main Skills Page for learning more about writing out equations.