When the U.S. dropped the atomic bomb, Little Boy, on the Japanese city of Hiroshima in 1945, it inflicted a huge number of casualties (~100,000 deaths) and completely leveled large portions of the city.
However, in the big scheme of nuclear warfare, this bomb and its cousin, Fat Man, which was detonated a few days later over Nagasaki, were relatively tame. Their explosive yield (15kt and 20kt, respectively) are a small fraction of the capability of the current American, Russian, or Chinese arsenals.
Fat Man
To give us all a sobering perspective on this, Alex Wellerstein created an interactive web site called NUKEMAP. Simply place the marker on your home (or any other location of your choosing), select a yield (or a specific bomb from the drop-down menu), and see the various damaging effects. Very interesting ...