A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission and or fusion. These reactions release a large quantity of energy from a relatively small amount of matter. A modern thermonuclear weapon weighing a little more than 1100kg can produce an explosive force comparable to the detonation of more than 1.1 million tonnes of TNT. Small nuclear device that are no larger than traditional nuclear devices can devastate an entire city by blast, fire and radiation. This is why nuclear weapons are considered weapons of mass destruction.
Only two nuclear weapons have been used in the course of warfare. Both of these instances were by the United States near the end of World War II. On August 6 1945, a uranium gun-type fission bomb code-named “Little Boy” was detonated over the Japanese city of Hiroshima. Three days later, a plutonium implosion-type fission bomb code-named “Fat Man” was exploded over Nagasaki Japan. These two bombings resulted in the death of 200, 00 people. However the lasting effects are still felt today.
Today, only a few nations possess nuclear weapons. The countries that acknowledge having nuclear weapons are the United States, the Soviet Union, the United Kingdom, France, the People’s Republic of China, India, Pakistan and North Korea. Some countries are believed to possess nuclear weapons but do not acknowledge them. The Federation of American Scientists estimates that there are 17, 000 nuclear warheads in the world as of 2012. 4,300 of them are considered “operational” and ready for use. 
"Federation of American Scientists: Status of World Nuclear Forces". Fas.org. Retrieved 2012-12-29.