The weapon of mass destruction of choice in the Atomic Age. So far, in human history, there have only been two recorded uses of nuclear weapons against enemy targets. These two events were Hiroshima (August 6th, 1945) and Nagasaki (August 9th, 1945) where the United States of America used atom bombs against the Empire of Japan in what would prove to be the conclusion of World War 2.
Nuclear weapons today have advanced far past the crude fission devices used in 1945 which worked by explosive compression of bits of plutonium or uranium to create a lump which exceeded critical mass. Hydrogen bombs were first developed by the U.S.A. in 1952. These fusion bombs worked by using the explosive power of fission bombs to set off a fusion reaction which fused isotopes of hydrogen to helium, multiplying the yield many times over.
A memorable hydrogen bomb test was done on Bikini atoll in 1954, a momentous event whose media effect was so large that the developer of a new two piece style of swimsuit for women named it the bikini.
Since then, there have not been any quantum leaps in nuclear weapon technology. A possible candidate is the development of the neutron bomb, a device that is designed to kill living organisms via lethal dose of radiation while having such a small explosive effect that it should leave most of the infrastructure intact.
We are, thankfully, still many years from developing even more destructive nuclear weapons which might include things like an anti-matter bomb. A single fusion warhead with enough yield to take out a medium sized city is still too heavy for a single person to carry. All bets are off if someone manages to build an anti-matter bomb.
Rewritten April 5th, 2002.