Fusion is a NUCLEAR REACTION

Nuclear reactions are different from chemical reactions because here, the transformations between species don't take place in their electronic structures, but in the nuclei themselves. (What are nuclei? )

The energy in a nuclear reaction comes from the transformation of mass to energy. Remember Einstein, E = mc²? According to this equation, a little mass can go a long way. (How long? )

In any nuclear reaction, nuclei are transformed in some way. Fission, the splitting of a large nucleus into two mid-sized nuclei, is currently the only nuclear reaction commercially used for it's energy output. See an example below on the left.

Fusion, the process of squashing two nuclei into one (shown below, right), releases much more energy. To find out why, see the next page.

Fission

Both animations courtesy of the University of Wisconsin Nuclear Reactor Tour Website