It is most famous for being the target of the first atomic weapon used in warfare, just prior to the Japanese surrender at the end of World War II. "Little Boy" was dropped on Hiroshima by the United States on August 6th, 1945 killing approximately 140,000 people. Three days later, the United States made the point that they were serious, by dropping "Fat Man" on Nagasaki, which was, so far, the last use of these weapons.
Hiroshima's current metropolitan population is estimated at 2,000,000. 
Itsukushima Shrine on Miyajima Island, which has one of Japan's most scenic views, is an interesting tourist site. It has a famous fireworks display.