So this topic is going to be a little edgy, because it is about arguably the single most devastating weapon invented by mankind. The Nuclear Bomb.
This snippet is from Wikipedia; http://en.wikipedia.org/wiki/Nuclear_weapon
A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission or a combination of fission and fusion. Both reactions release vast quantities of energy from relatively small amounts of matter. The first fission ("atomic") bomb test released the same amount of energy as approximately 20,000 tons of TNT. The first thermonuclear ("hydrogen") bomb test released the same amount of energy as approximately 10,000,000 tons of TNT.
How nuclear bombs work isn't too complicated and has been done to death, but let's talk about the effects of these weapons of mass destruction. These effects are listed in the order that they arrive.
Effect 1: Light
When the bomb goes off, the first thing that happens is a blinding flash of light (literally blinding). The cause of this is the initial burst of energy from the explosion (imagine a firecracker's little flash of light and multiply it by more numbers than you brain is comfortable with). The light is SO bright that it can actually cause damage to you on it's own, resulting in burns and other horrifying disfigurements. There were 7 confirmed cases of the light causing a secondary reaction that was also fatal. Curiosity (see effect 3).
Effect 2: Electromagnetic Pulse
The electromagnetic pulse generated by a nuclear bomb is one of the largest man-made EMP's to exist. The EMP pretty much destroys anything electrical. It is electromagnetic radiation caused by the reactions happening in the initial explosion of a nuclear bomb combining with the planet's electromagnetic fields. This isn't so much deadly to humans as it is to their cell phones and gaming consoles. The nuclear bomb itself DOES NOT CONTAIN ENOUGH EXTRA CHARGES to create an EMP on it's own. Instead it relies on the Earth's ionosphere to generate this effect. This means that without the presence of electromagnetic fields (i.e. ionosphere) a nuke would not be able to produce an EMP. In other words, unless you are close to a planet or a sun, and close enough to critically damage whatever spaceship you are on, your nuke will not be able to pop covie shields.
Effect 3: Sound Wave
The light travels faster than sound, and so you see the giant flash first. This may cause you (and in the case of 7 people, did) to want to look out your window to see what the light was. Then the sound wave hits, causing your window to shatter so violently that it causes extreme harm to anyone nearby, turning literal the phrase "curiosity killed the cat". Satisfaction won't be bringing it back in this case though. The sound wave is strong enough to shatter glass, cause damage to your internal organs (which, with what's coming next is the least of your problems if you're close enough for that to happen), and blow out your eardrums.
Effect 4: Blast Wave
This is the one that arguably causes the second most damage. The blast wave of the explosion, for all intents and purposes, disintegrates everything in it's path. It has such an immense amount of heat and pressure that nothing that we have yet to build anything to protect us from "ground zero". We are talking about enough heat to flash-fry anything in it's radius, which is quite extensive. The blast wave is so powerful that it creates "shadows". When an object gets in the path of the blast wave, it shields the surface of what is behind it from the extreme heat. This creates the most macabre shadow in the history of man. The shadow is actually the original color of the object that was shielded, while everything around the shadow was colorchanged due to the extreme heat.
Effect 5: Radiation
This effect is more devastating than the rest of the bomb in the minds of most. It happens during the blast wave, but the effect isn't pronounced until all is said and done. There are two types of radiation caused by nuclear weapons. Initial Radiation, which is the radiation generated by the explosion itself, and Residual Radiation, which is radiation caused by the initial radiation. Initial Radiation is the "quick" killer, as it affects things immediately (generally resulting in radiation poisoning). Residual Radiation does things like cause cancer, kill livestock and crops, and a slue of other things that radiation is known to do. Residual Radiation can stick around for quite some time. Thankfully in the only two instances of human beings being in the path of the monstrous weapons the weapons were detonated in the air, causing less residual radiation to occur. If they had been detonated at ground level, the two cities would likely have either just become habitable again or still be uninhabitable. There also would have been additional serious consequences, which brings us to the next effect.
Effect 6: Fallout
No, not the awesome game. Also referred to as "black rain" nuclear fallout is separated in this topic because it has a much more devastating effect than initial radiation. Fallout is caused when a nuclear explosion sucks up water, plants, ground, and pretty much everything within a certain radius, radiates it, and flings it into the sky. Irradiated dust and debris become clouds and irradiated water becomes rain. These "black rain" clouds then travel along the wind currents until condensation takes it's toll on the water, and it begins to rain. This rain has the exact opposite effect than normal rain, causing the death of livestock, plants, and people due to radiation illness. That's not the worst part though. Theoretically, as long as the dust and debris stay in the air, it can catch more condensation and create more black rain. It can do this until the cloud dissipates and floats back to the ground, irradiating everything it lands on. The consequences of fallout are very severe can last a very long time.
That, ladies and gentleman, is a look at what a nuclear weapon can do. Start to finish. A nuclear weapon is deadly to EVERYTHING. Inorganic material can be vaporized by the blast, organic material can be destroyed by the blast and radiation, and electronic material can be destroyed by the EMP generated (as long as there is an electromagnetic field available to the nuke at the time, that is).
This makes it the potentially deadliest weapon created by mankind thus far.