Short answers, pulled from the story.
The United States dropped an atomic bomb on Hiroshima on the 6th of August 1945 and a second on Nagasaki on the 9th of August 1945, during the final days of World War II. Japan announced its surrender on the 15th of August, six days after Nagasaki.
The bombings killed between 150,000 and 246,000 people, most of them civilians. Roughly half the deaths occurred on the first day, with many more dying over the following weeks and months from burns, radiation sickness, illness, and malnutrition.
The Hiroshima bomb was Little Boy, a gun-type fission weapon using uranium-235, and it held about 64 kilograms of uranium-235. The Nagasaki bomb was Fat Man, a more complex implosion-type weapon using plutonium-239. Both were produced by the Manhattan Project.
Hiroshima was an embarkation port and industrial center with a major military headquarters, and nearby mountains were expected to focus and increase the blast. Nagasaki was added on the 25th of July 1945 in place of Kyoto, as a major military port and one of Japan's largest shipbuilding centers. Targets were large urban areas that also held significant military facilities.
Secretary of War Henry L. Stimson asked Groves on the 30th of May 1945 to remove Kyoto because of its historical, religious, and cultural significance. When Groves pointed to its military and industrial value, Stimson took the matter to President Harry S. Truman, who agreed, and Nagasaki was later put on the list in its place.
The B-29 Enola Gay, piloted by Colonel Paul Tibbets and named after his mother, dropped Little Boy on Hiroshima on the 6th of August 1945. The B-29 Bockscar, flown by Major Charles Sweeney's crew, carried Fat Man on the 9th of August with Kokura as the primary target and Nagasaki the secondary.
Officials and scientists feared a demonstration would fail, since the Trinity test had used a stationary device rather than an air-dropped bomb, and only one bomb would be available at the start of August. They also worried a non-lethal demonstration would sacrifice the shock value and that Allied prisoners of war might be moved to the site and killed. The Scientific Advisory Panel concluded on the 16th of June 1945 that it saw no acceptable alternative to direct military use.