With nuclear fission, a large atomic nucleus (such as a uranium nucleus) breaks apart into smaller nuclei, and energy is released. With nuclear fusion, small atomic nuclei (such as hydrogen) join to become larger nuclei, and energy is released. Fusion of hydrogen releases much more energy than any other type of either fusion or fission. Note that the dividing line between heavy nuclei and light nuclei is the iron nucleus, which is at the perfect point of nuclear stability, so that neither fusion nor fission of iron nuclei would release any energy.
