Fusion releases energy. The energy released is linked to Einstein's famous equation, E=mc2.
For any fusion reaction to take place it is required to bring the nuclei so close together that nuclear forces become essential and glue the nuclei together. The nuclear force only works over incredibly small distances and has to counteract the electrostatic force in which the positively charged nuclei repel one another. For these reasons fusion most easily takes place in a high temperature environment, high density.
On the planet, nuclear fusion was initially reached in the explosion of the Hydrogen bomb. In a nondesctructive way, fusion has also been reached in various experimental devices targeted at studying the possibility of generating energy in a managed fashion.
Every single day, space exhibits us that nuclear fusion can generate considerable sums of energy. The only issue is that this phenomenon occurs a long way from Earth, in circumstances that are not found on our world. Science is endeavoring to reproduce the circumstances required for fusion.
A Natural Phenomenon in the Sun. Nuclear fusion contains combining two light atomic nuclei to make a heavier nucleus. This releases a massive amount of energy. The Sun converts 600 million tons of hydrogen into helium per second, releasing considerable sums of energy.
The Sun changes 600 million tons of hydrogen into helium each second.
Throughout fusion, hydrogen forms a plasma (the state of matter when warmed to a very higher temperature). The atomic nuclei within the plasma shed their electrons and the forces of repulsion among these are extremely high. However, some fuse and change into helium, releasing large sums of energy. On the Sun, the probability that 2 hydrogen nuclei will fuse is very low but this is offset by the large numbers of nuclei present.
The key advantages of nuclear fusion are: The incredible sum of energy available, about 5 times more energy for each kilogram than fission fuels and about ten million times more energy for each kilogram than coal. The simple availability of fuel the currently proposed research reactors and demonstatration power plants will use a combination of tritium and deuterium gases (forms of hydrogen).Deuterium is simple to extract from seawater and tritium can be produced inside the reactor from lithium, a fairly easily accessible metal. Safety, unlike a fission plant, no runaway reaction is possible. The whole reactor vessel (about 840m^3 for ITER, the greatest reactor in the planet when it is finished) contains lower than a gram of fuel at any time and the plasma exists in such a delicate and carefully managed state that any disturbance or disruption will merely cause it to hit the walls of the reactor cool rapidly to the point in which fusion is no more possible.