The effects of an atomic bomb are very deadly.  The explosion will totally disintegrate almost anything close to ground zero.  Within many miles of ground zero, everything that is capable of burning, will burn.  Another after effect is a high pressure pulse that moves slightly faster than the speed of sound outward for many miles, knocking down buildings and people.  This is also known as a shock wave.  This is usually accompanied by very powerful winds that are much faster than those in hurricanes.  The oxygen in the air will be burned up by the fireball created from the blast that can reach up to diameters of 300 meters.  Sometimes firestorms are created, much like a large forest fire which sucks in air from around and makes anything flammable in the surrounding area light up in flames.  The atomic explosion also generates a vast amount of radiation, killing many living cells (diagram).  The survivors that live through the deadly blast and the tremendous heat and shock wave do not live through the radiation poisoning.  Luckier victims die right away but some suffer a long painful death that sometimes lasts for weeks, months or even years.
    Other than just damage from the blast effects, a full scale nuclear launch could create a significant change in the atmosphere, changing the weather.  This has become known as nuclear winter.  The nuclear weapons launched would knock enough dust from the ground into the air to block off sunlight for months.  This would produce sub-freezing temperatures and kill plant life, and animals.  The nuclear weapons could also damage the ozone layer, permitting even further damage to organisms and plants after the dust has dispersed.