The bomb which was dropped on Hiroshima was a gun triggered fission bomb. When the fission a-bomb hits the ground and explodes, it smashes a reactive particle into a sizable amount of Uranium (splitting the Uranium atoms). When this happens a chain reaction starts; each atom going through nuclear fission and letting out a large ammount of energy for each reaction. When this kind of explosion goes off it takes a while to slow down and then to stop.
An explosion this big has the power to decimate the immediate territory, and also the radioactive remnants of the explosion can hang around and drift to nearby areas, in this case Asia. Amazingly enough there were humans and objects which survived (though in very bad shape).
