How does a nuclear bomb explode?

[NewBie911]

ok , it might be a silly question .. but i just want to know how does the nuclear bomb react when her rockets touch the ground .. for sure the nuclear bomb isn't like the normal bomb or the dynamite that needs a spark to exploid ( excuse me I am not that good in physics :v )

 

[mfb]

Typically nuclear weapons are ignited while they are above the ground, that increases the damage. For the explosion itself it does not matter where the bomb is. A pile of uranium or plutonium (of the right type) that is large and compact enough will make a chain reaction, where one fission triggers more than one subsequent fission process until a large part of the nuclei are fissioned and a huge amount of energy has been released.
When stored, you need the uranium/plutonium in the bomb - but you have to keep it separated so it doesn't start a chain reaction. To "ignite" a weapon, chemical explosives are triggered, they push the material together and compress it. When the material is maximally compressed, a few neutrons are shot into the material to start the chain reaction.
Fusion weapons (thermonuclear weapons) use a fission bomb to heat up the hydrogen that starts fusion once it is hot enough.

Wikipedia explains all that in more detail.