This poisoning is caused by Tetrodotoxin(TTX), which is an extremely potent neurotoxin present in the bodies of fish in the Tetraodontiformes and fugu families - 1 mg of this substance is enough to kill an adult human. These fish have an hereditary immunity to TTX and use it on their prey. However, certain pufferfish are considered delicacies in Japan (as sushi). Chefs who wish to prepare fugu sushi must be licensed by the government to do so. Fugu poisoning has about a 50% mortality rate.

The stages of tetraodon poisoning in humans
numbness (mouth) > loss of feeling in extremities > sense of floating > headache, nausea, etc (what coby said) > reeling > paralysis > respiratory distress > still conscious, possibly lucid, but completely paralyzed > death. This may take between 20 minutes and 8 hours.