The A-bomb, or atomic bomb, has been used only twice in war, at Hiroshima and Nagasaki, and both times it has totally destroyed a large city in a single explosion.

Strictly speaking, the term atomic bomb includes the H-bomb, which also uses atomic power. But in general usage, atomic bomb is reserved for earlier weapons that work by nuclear fission, that is, splitting atoms. The hydrogen bomb works by nuclear fusion - joining small atoms together to make larger ones. Both fission and fusion release huge amounts of energy and radioactivity.

Nuclear fission