Liquid nitrogen is a liquefied gas with a boiling point of -196C. When it changes to vapour at anything above that temperature it takes in an awful lot of heat and expands to about 700 times its liquid volume.
It is dangerous because it is so cold and because it expands so rapidly.
Liquid nitrogen will cause very rapid cold damage to flesh, and if the volume expands in a confined space it will expand and force out the oxygen, the body only detects when carbon dioxide arises not when the oxygen level is low and so it is very insidious.
Because the liquid nitrogen basically explodes in to its gas form when it comes in to contact with water because of the massive heat exchange it would cause rapid distension and may even explode in a confided space like the stomach.
It is has become increasingly popular in kitchens and is used to flash freeze foods. When used in this way on a water containing substance there is no formation of ice crystals, so the material remains a better texture.
But because it is so dangerous it must always be treated with caution and never ingested.