Nuclear weapons and nuclear war, are perhaps the most feared weapons of destruction known to man. The pilot of the "Enola Gay", the aircraft used to drop the first nuclear bomb in anger was quoted as saying "My God! What have done!?" after seeing the detination over the city.
The first nuclear weapons were relativly simple compared with modern nuclear weapons. They used 'simple' chain reactions of high purity Uranium-235 or Plutonium-239 to create a large explosion and massive discharge of heat and radiation. The reaction is triggered by forcing a small amount of Uranium into a larger one at very high speeds, with high energy and under extreme pressure. This causes the two pieces of Uranium to weld together inside the bomb, causing a nuclear chain reaction.
To prevent the accidental detonation by a stray neutron, and to protect those around the bomb during transit, a lining of Uranium-238 is used. Uranium-238 is nonfissionable, and has a high neutron capture cross section, stopping neutrons from getting in.
Plutonium bombs are similar to Uranium bombs in their somewhat simplistic design. Again a very pure sample of Plutonium-239 is needed for a nuclear reaction to occur. However, the detonation mechanism is slightly different. A radioactive source is used to release a neutron which starts the chain reaction. The second nuclear bomb dropped was of this type, and was dropped on the Japanese port city of Nagasaki on August 9th 1945, bringing to an end World War 2.
The yields of these weapons were equivilent to 20,000 Kilotonnes of TNT. The damage caused is greater than simply the initial explosion. When the bomb exploded at Hiroshima it was triggered to go off at around 2,000 feet, meaning the fallout from the bomb (the residual radiation and byproducts of the chainreaction), would spread out over a large geographical area. Common constituents of nuclear fallout are spent nuclear fuel, unspent nuclear fuel and isotopes of other atoms which have become radioactive after absorbing neutrons from the explosion.
Most of the destruction is caused by the initial blast damage. Most buildings, except the strongest, will be almost completly destroyed. As the bomb explodes, radioactive winds are created, often travelling at several hundred miles per hour - This is known as the "Air Burst".
As stated before a lot of EM radiation is released during the explosion. This is not just limited to X-Ray and Gamma, but also UV, IR and visible. This causes many people who are not killed by the initial thermal effects of the bomb, to have severe burns similar to sun burn.