Fusion is a process by which the nuclei of two atoms are combined or "smashed" together at high speed and high temperatures. This results in the creation of a completely different kind of element with different chemical composition. Scientists have discovered that fusion is a basic process that takes place throughout the universe -- most prominently on the sun, in which atoms of hydrogen are fused together at temperatures exceeding 40 million degrees.
Fusion produces a counter-intuitive outcome. The element that results from the reaction is less than the sum of the parts. In other words, the object that comes out weighs less, not more, than the individual origin elements. There's a simple explanation for this. The individual particles are more tightly bound after the reaction than they were before. The excess mass of the objects is released in the form of energy. This is exactly what Einstein described in his equation E=mc2.
Scientists have identified two atoms of hydrogen as having great potential in fusion: Deuterium and Tritium. Both can be found in large quantities in the Earth. The combination of these two atoms produces an atom of helium as well as a neutron, which is the excess energy. If harnessed carefully, in a controlled fashion, this neutron would have many beneficial applications. As example, as a heat source. The key is being able to control the reaction.
The problem is physicists have not been able to harness the excess matter in a fusion reaction to provide electricity and heat on Earth. The potential is vast and most vividly demonstrated in the form of the nuclear weapon. Nuclear weapons are a form of fusion. But with the bomb, the energy is all released at once, with no attempt to control it. Experiments are continuing to find ways of creating harnessed fusion reactions for the good of society.