Nuclear power is any nuclear technology designed to extract usable energy from atomic nuclei via controlled nuclear reactions. The only method in use today is through nuclear fission, though other methods might one day include nuclear fusion and radioactive decay (see below). All utility-scale reactors heat water to produce steam, which is then converted into mechanical work for the purpose of generating electricity or propulsion. In 2009, 15% of the world's electricity came from nuclear power.
A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission or a combination of fission and fusion. Nuclear weapons are considered weapons of mass destruction, and their use and control has been a major aspect of international policy since their debut.
In the history of warfare, two nuclear weapons have been detonated offensively. On the morning of 6 August 1945 the United States dropped a device code-named 'Little Boy' on the Japanese city of Hiroshima. Three days later, the United Stated dropped a second bomb; code named 'Fat Man' it was dropped on the city of Nagasaki, Japan. These bombings resulted in the immediate deaths of around 80,000 people (mostly civilians). A further 40,000 people are believed to have died from long-term effects of radiation poisoning.
Information drawn from Wikipedia pages 'Nuclear Power' and 'Nuclear Weapons'.