Jellyfish are members of Phylum Cnidaria, Class Scyphozoa or true jellyfish. They are "cup-shaped" animals. Scyphozoans are organisms with tentacles that can sting their prey. They can be found in oceans all over the world. Jellyfish have special nematocysts, or stinging cells located along their tentacles, which act like harpoons and can sting prey, injecting a venom in prey or humans. Cubozoa, or box jellies can be particularly dangerous. Hundreds of people have been killed by jellyfish stings in the past century. Some large jellyfish are considered to be a delicacy and are consumed in parts of Asia and are commercially fished.
its sting is quite harmful to human compared to an insect bite. furthermore, stings from jellyfish can also result in severe pain and there are some cases where it leads to fatal.
there are few precautions that you should apply if you were stung by jellyfish. do not rub the area, splash the area with water. this is because, you will make it worse by rubbing the area as it will trigger more stinging cell. you can pour something acidic on the wound like some soft drinks (coke) ,vinegar, etc.