What does nuclear fusion do?

Nuclear fusion is the fusion of two nuclei (atoms) which, when combined, form a highly unstable element or isotope, which almost immediately breaks up, producing energy and more, smaller nuclei to continue this chain reaction. The energy given of can be harnassed as heat to heat water into steam to turn a turbine to produce electricity, in which case the reaction would be controled, or it can be allowed to go out of control, in which case it would produce a massive explosion.