Fusion is the process whereby smaller elements come together to form larger ones.

For example, hydrogen atoms can fuse together to form heavier atoms such as helium. The process would release a lot of heat This is what powers the sun.

Cold fusion is the suggestion, not proved so far, that under certain controlled conditions, very small numbers of atoms can be made to undergo fusion (typically in a beaker) in such a way that small amounts of heat are released in a controlled way. Since it would not be allowed to get very hot, it is called "cold" fusion.

The basic idea of fusion is that if you can take two light atoms and force them together to become a larger atom you can get some extra energy in the process. One example is taking two deuterium atom (a type of hydrogen with each atom having one proton and one neutron) and fusing them into a helium (2 protons + 2 neutrons) nucleus. Although fusion is what keeps our Sun shining, it is difficult to keep a fusion reaction going on Earth because it requires a lot of energy to force the positively charged deuterium atoms together to form helium. This is like someone telling you that if you give him one million dollars that he will give back one million plus one hundred dollars. It would be great to get that extra one hundred dollars but first you have to come up with the million dollars which might not be so easy.

The idea of cold fusion is that maybe there is a way to fuse two deuterium nuclei without having to pour in all the initial required energy. One thought was having a chunk of palladium, which adsorbs hydrogen very effectively, and keeps adding more and more hydrogen to the palladium until hydrogen naturally fuses. Seems like a great idea but is still very controversial. For more info see:

cold fusion

There do seem to by types of cold fusion that are proven to work but none that are practical so far for general energy generation.

Fusion refers to the nuclear reaction by which (comparatively) small atomic nucle, such as those of hydrogen or deuterium, combine to form heavier atomic nuclei, such as those of helium.

Fusion releases energy for the combining of elements lighter than iron; for elements heavier than iron, it takes energy (the opposite process, fission, releases energy). Despite the tremendous amount of energy that it produces, however, nuclear fusion requires tremendous temperatures and pressures to occur, so high that it would cause the system undergoing nuclear fusion to explode (as in "hydrogen bomb"), thus cooling it off and stopping the reaction.

Nuclear fusion has only been observed sustainable in the case where the system undergoing the fusion is so massive that its own gravity keeps it from blowing itself apart; this is what stars are (including the Sun).

Cold fusion is a hypothetical idea that it might be possible to sustain nuclear fusion under safe, low temperatures and pressures. The original proposed mechanism by which this might happen has since been discovered to be a hoax. This has not stopped many physicists and nuclear engineers from searching for a different mechanism by which it could be done. So far nobody has succeeded in finding a way, and we are not certain if there is a way. The reason for the search, however, is that if cold fusion were possible, and we could do it, then it would solve the world's energy crisis with an almost unlimited supply of energy: the fuel source is hydrogen, which is about 90% of the Universe.