The Atom Bomb is a super weapon designed by scientists in the late 1930's through the 1940's during the second world war. It has a complex firing mechanism that involves splitting the nucleus of a heavy atom like uranium 235. When this happens a certain amount of energy is released and an equivalent amount of mass disappears. That energy is the energy that powers the bomb. On a pound-for-pound basis the U-235  in the bomb can release on the order of a million times as much energy as TNT. In 1945 the U.S air force sent a b-29 bomber the Enola Gay to Japan to drop a nuclear bomb named "Little Boy" "A bright light filled the plane," wrote Lt. Col. Paul Tibbets, the pilot of the Enola Gay, the B-29 that dropped the first atomic bomb. "We turned back to look at Hiroshima. The city was hidden by that awful cloud...boiling up, mushrooming." For a moment, no one spoke. Then everyone was talking. "Look at that! Look at that! Look at that!" exclaimed the co-pilot, Robert Lewis, pounding on Tibbets's shoulder. Lewis said he could taste atomic fission; it tasted like lead. Then he turned away to write in his journal. "My God," he asked himself, "what have we done?" (special report, "Hiroshima: August 6, 1945")
note: Paul Tibbets was Colonel, not "Lt. Colonel," when he was the pilot of the Enola Gay. http://webpages.charter.net/jaskaggs/AEII/Hiroshima.htm Another bomb was dropped on Nagasaki 3 days later. It was an even more powerful bomb but caused less casualties because of the location of the city. Hopefully no more nukes will be used in war again.