Depends on the delivery system, the kind of nuclear device and who built it. I will not go in depth about the details, I will however touch upon the basics. Type of Nuclear Device: An "atomic bomb" is considered a straight fission type bomb - like the ones dropped on Hiroshima and Nagasaki. One way to build such a bomb is to have enough mass of fissile material come together reaching a critical mass. They have a fissile material (uranium, plutonium, etc) which is separated or divided into smaller pieces. When all of those pieces are together there is a 'critical mass' of fissile material, the exchange of subatomic particles accelerates to 'explosive' proportions. Another way is to have a smaller mass of fissile material surrounded in a 'ball' of conventional explosives (like C4 or TNT) the conventional charges are all 'shaped' meaning they are designed to concentrate their force inward and equally on a sphere of fissile material. The 'compression' upon the explosion of the conventional explosives is enough to compress the fissile material to jump start its own nuclear reactions leading to explosion. There are variations on how this is done exactly. There are 'seeded' or 'salted' atomic bombs (at least in theory) where a complementary material like cobalt is introduced to the core that complimentary material causes different reactions at the time of fission (then the fissile material explodes) that can lead to various affect. In the case of cobalt the release of greater amounts of radiation takes place. Cobalt Bomb: http://en.wikipedia.org/wiki/Cobalt_bomb Hydrogen or thermonuclear bombs take the atomic bomb one step further. Usually its is a three layer type of bomb. A conventional or chemical explosive surrounds a fissile material explosive (like plutonium) which surrounds a core of say hydrogen. The conventional explosives compress the fissile material to the point of a runaway reaction or detonation - that in turn compresses and hats up the hydrogen core leading to fusion of the hydrogen atoms. the fusion of hydrogen releases far much more energy than the fission of say uranium. Most bombs are now designed to either ride in a plane as a missile warhead or a 'drop' bomb. Or they sit atom rocket engines commonly called missiles. Depending on the device and who makes it, and the amount of safety devices installed to prevent accidental detonation, a code is sent too the device prior to its launch/release that arms the device. At the moment of detonation what takes place depends on what kind of device we are talking about. I believe that one of the bombs used at the end of WWII was a two 'plate' type bomb where two sub critical masses of fissile material were slammed into each other. The other used shaped charges around a fissile core. Modern devices used shaped charges mostly, shaped conventional surrounding shaped fissile surrounding a hydrogen core.

The atomic bomb was researched during WWII and was known as the Manhattan Project. An atomic bomb is a powerful nuclear weapon fuelled by the splitting, or fission, of the nuclei of specific isotopes of uranium or plutonium in a chain reaction. The strength of the explosion created by an atomic bomb is on the order of the strength of the explosion created by thousands of tons of TNT. An atomic bomb must provide enough plutonium or uranium to reach critical mass, the mass at which the nuclear going on inside the material can make up for the neutrons leaving the material trough its outside surface. Usually the plutonium or uranium in a bomb is separated into different parts so it does not reach critical mass until the bomb is set to explode. At that point, a set of chemical explosives or some other mechanism drives all the different parts of uranium or plutonium to produce a critical mass. After this happens there are enough neutrons bouncing around in the material to create a chain reaction of fissions. In the fission reactions, collisions between neutrons and uranium or plutonium atoms cause the atoms to split into pairs of nuclear fragments, realising energy and more neutrons. Once the reactions begin, the neutrons realised by each reaction hit other atoms and create more fission reactions until all the fissile material is exhausted or scattered.