There are approximately 2,000 species of jellyfish worldwide yet only less than 100 or so are actually thought of as dangerous and can cause harm to human and animal life. It is true that jellyfish can still sting even after they have died. This is because jellyfish do not consciously sting. Each of their tentacles can be equipped with stinging cells that automatically release venomous barbs when the cells come into contact with an object. This is done as a defense mechanism to keep jellies safe from predators. It does not work on sea turtles however. Interestingly enough these turtles can eat jellyfish without getting stung or poisoned.
What may come as a surprise to most is that jellyfish can even still sting you when they have died, washed up on a beach and dried up. The tentacles (even if they are no longer connected to the jellyfish) can still sting and it is for this reason that these jellyfish or their parts should never be picked up or stepped on. Some jellyfish are more poisonous than others; however all can cause quite a bit of pain if they sting an individual.