A nuclear weapon is a weapon which derives its destructive force from nuclear reactions of either nuclear fission or the more powerful fusion. As a result, even a nuclear weapon with a relatively small yield is significantly more powerful than the largest conventional explosives, and a single weapon can destroy or seriously disable an entire city.
In the history of warfare, nuclear weapons have been used only twice, both during the closing days of World War II. The first event occurred on the morning of 6 August 1945, when the United States dropped a uranium gun-type device code-named "Little Boy" on the Japanese city of Hiroshima. The second event occurred three days later when a plutonium implosion-type device code-named "Fat Man" was dropped on the city of Nagasaki.
The use of the weapons, which resulted in the immediate deaths of at least 200,000 individuals (mostly civilians) and about twice that number over time, was and remains controversial — critics charged that they were unnecessary acts of mass killing, while others claimed that they ultimately reduced casualties on both sides by hastening the end of the war.(See Atomic bombings of Hiroshima and Nagasaki for a full discussion.)

Since that time, nuclear weapons have been detonated on over two thousand occasions for testing and demonstration purposes, chiefly by the United States, Soviet Union, United Kingdom, France, People's Republic of China, India, and Pakistan.

The history of nuclear weapons chronicles the development of nuclear weapons—devices of enormous destructive potential which derive their energy from nuclear fission or nuclear fusion reactions—starting with the scientific breakthroughs of the 1930s which made their development possible, continuing through the nuclear arms race and nuclear testing of the Cold War, and finally with the questions of proliferation and possible use for terrorism in the early 21st century.

The first fission weapons ("atomic bombs") were developed in the United States during World War II in what was called the Manhattan Project, at which point two were dropped on Japan. The Soviet Union started development shortly thereafter with their own atomic bomb project, and not long after that both countries developed even more powerful fusion weapons ("hydrogen bombs"). During the Cold War, these two countries each acquired nuclear weapons arsenals numbering in the thousands, placing many of them onto rockets which could hit targets anywhere in the world. Currently there are at least seven countries with functional nuclear weapons. A considerable amount of international negotiating has focused on the threat of nuclear warfare and the proliferation of nuclear weapons to new nations or groups.

The declared nuclear powers are United States, Russia, United Kingdom, France, People's Republic of China, India, and Pakistan. Various other countries may hold nuclear weapons, but they have never publicly admitted possession, or their claims to possession have not been verified. For example, Israel has modern airborne delivery systems and appears to have an extensive nuclear program with hundreds of warheads (see Israel and weapons of mass destruction); North Korea has recently stated that it has nuclear capabilities (although it has made several changing statements about the abandonment of its nuclear weapons programs, often dependent on the political climate at the time) but has never detonated a weapon and its weapons status remains unclear; and Iran was accused by a number of governments of attempting to develop nuclear capabilities, though its government claims that its acknowledged nuclear activities, such as uranium enrichment, are for peaceful purposes.

source: http://www.shundahai.org