In a plasma the electrons are stripped from the atoms, creating a substance that resembles a gas but that conducts electricity. The stars, in particular, are giant balls of plasma. The earth is an unusual place where matter exists in the other three forms. Plasmas represent a fundamental field of study, with many astrophysical and terrestrial applications. Fusion is a nuclear process, not to be confused with the fission process that powers all present nuclear reactors.
Fusion is quite different from fission.
Fission uses as its fuel heavy elements like uranium which are split into lighter elements with the production of undesirable radioactivity. By contrast, fusion combines light elements like isotopes of hydrogen to form heavier elements, producing relatively little radioactivity. Fusion fuel is abundant and inexpensive. Each gallon of water has about an eighth gram of deuterium, which if burned in a fusion reactor, would produce as much energy as 300 gallons of gasoline. A fusion reactor is also much safer than a fission reactor because very little fuel is in the reactor at a time.