Energy is released when the nucleus of an atom changes. A change in a nucleus is called a nuclear reaction. Three kinds of nuclear reaction can be a source of energy: fission, fusion and radioactive decay.
In fission, a large, unstable nucleus such as uranium-235 splits into two smaller nuclei. In fusion, two small nuclei, such as hydrogen nuclei, join together to make one larger nucleus. In radioactive decay, an unstable nucleus emits radiation to become more stable. Each of these releases energy.
The energy released per kilogram of fuel is huge compared with burning a fuel, because the forces inside a nucleus are far stronger than the forces between atoms. Nuclear power stations use fission. Stars such as the Sun are powered by fusion. Radioactive decay produces heat inside the Earth and can power small devices such as space probes.