Certain species of jellyfish can in fact be deadly to humans. However most are harmless but may be known to sting a little. There are a few jellyfish that can kill a human being in only a matter of minutes. Among them is the box jellyfish that can kill an adult human in a mere 4 or 5 minutes.
Jellyfish are usually passive drifters who use their tentacles to catch small prey, however they can pack on quite a nasty nip that will result in excruciating pain depending on the species. The box jellyfish (mentioned above) is actually one of the most venomous marine creatures in the world and can string and kill a man within minutes. Most fatalities occur just following a brush with a toxic jellyfish like this.
If an individual is going to be swimming in an area where there are known jellies, it is recommended that they wear a sting suit. However, if you do happen to get stung somehow it is important to get out of the water immediately and apply vinegar to any stings remaining embedded in the flesh. This should be done in order to remove the venom and then it should be brushed off or lifted off using a stick, or something of the like.
The best word of advice is to stay as far away from jellyfish as possible! This is the best way to ensure your safety.