Well, the explosives do it. An atomic-bomb requires using a large sum of chemical explosives (I believe that C-4 is capable of doing the trick) to spark the atomic chain-reaction. That reaction sustains itself for a length of time, generating a massive amount of energy, and a LOT of waste product. The bombs on Hiroshima and Nagasaki used only a miniscule fraction of the atomic energy available in the plutonium (actually, that much material, if the chain reaction was 100%, would have destroyed all life on this little rock, boiled our ocean, turned the surface of this planet into liquid, blown the atmosphere into space, and possibly knocked us out of orbit).
Now, a nuke is different. It involves the same principle, but uses the atomic chain reaction to start a fusion reaction. Same amount of mass, and many times over more power. A typical nuke is less than half the size of those two infamous atomic bombs, and will do hundreds of times more damage. Hiroshima and Nagasaki had survivors and buildings standing. One launch from a nuclear attack sub... and there would only have been irradiated ash.
