Hiroshima is currently most famous, of course, for the fact that it was the first city to be atom-bombed. The city was completely destroyed by the bomb, and so everything you see around you has been built after WWII.
The major attraction of Hiroshima is the Peace Memorial Park. This includes the A-Bomb Dome, the park and the museum. See a separate entry for more information on these.
Hiroshima has a system of street cars that operates rather frequently. One terminal is at the main train station, so this is a great way to connect with JR/bullet trains. Get on the street car, and pay the conductor when exiting. There are maps of the street car lines inside the cars, with english station names on them. The major tourist stops are also announced in english.
Hiroshima is about 4 to 4.5 hours from Tokyo by bullet train.