On August 6, 1945, exactly 62 years ago, the nuclear weapon Unit L-11 nicknamed Little Boy was dropped on the city of Hiroshima by the crew of the American B-29 Superfortress bomber Enola Gay, directly killing an estimated 70,000 people. Approximately 69% of the city's buildings were completely destroyed, and 6.6 percent severely damaged as the United States became the first and only country ever to use an atomic weapon in warfare. In the following months, an estimated 60,000 more people died from injuries in Hiroshima, and hundreds more from radiation.
Three days later, on August 9, 1945 the port city of Nagasaki was destroyed by a second atom bomb nicknamed Fat Man with the ultimate loss of 140,000 lives. The results were that Japan surrendered and World War II was ended. Here is some additional information according to The Manhattan Project - An Interactive History (the emphasizes were made by me):
Development of the bomb followed two paths, one using uranium-235, which occurs naturally, and the other man-made plutonium. In the end, both were built and used: The uranium-based "Little Boy" was dropped on Hiroshima, while the plutonium-based "Fat Man" laid waste to Nagasaki.
How heavily populated cities came to be chosen as the targets remains a matter of controversy. The scientists involved in developing the bomb favored demonstrating their weapon to the Japanese in an isolated area but military and political planners rejected the idea, arguing that the shock of total destruction would have a more profound impact.
The United States maintains to this day that the decision to drop the bomb was made primarily to avoid the necessity of invading the Japanese home islands, an undertaking that would have resulted in enormous casualties on both sides. But that argument ignores the deterioration of Japanese resolve by that point in the war. Although the emperor's government rejected the Potsdam Declaration in late July, which called for an immediate and unconditional surrender, the Japanese had been sending out peace feelers through the Soviet Union, and early signs of starvation, even on the main island, were apparent.
Many historians believe that the real U.S. motive for dropping the bomb was to end the war quickly before the Russians could become involved, thereby denying them a postwar stake in the Pacific -- and, by practical example, to send a message to Stalin.
Whatever the reasons, the bombs were dropped and most of the scientists involved in the Manhattan Project later expressed remorse for what they had wrought.
The above photograph (source: About History) shows Comdr. A.F. Birch, numbering LB (Little Boy) unit L-11, before loading on trailer in Assembly Bldg. #1. Unit L-11 was the one dropped on Hiroshima. Dr. Ramsey standing nearby. (August 1945).
Here is a 10 min video piece from BBC's Hiroshima.