I am sure most of us have seen jellyfish of some description washed ashore on the beach or even when snorkeling or swimming, but further then that most people don't know a whole lot. The fact is that although they are often synonymous with such species as the Portuguese man o war, which of course is very dangerous to be in the water near, most jellyfish are in fact harmless and don't have stings strong enough to either pierce human skin or poison strong enough to do us any harm. There are in fact over 200 species of true jellyfish as well as hundreds of their close relatives, such as the Portuguese man o war, which is often mistaken for a jellyfish although ti differs because of its air pocket and the fact that it has a different type of toxin to other types of jellyfish.
Box jellyfish also have visual systems which allow them to actually hunt their prey and swim towards it, rather than just floating with the pull of the tides as man jellyfish do, which is why a lot of them tend to be washed up on the beaches. The vast marority of them are transparent and are almost invisible when in the water. This is partially because of the fact that their bodies are made up of 98% water already, and the fact that they are invertebrates, meaning they don't have a backbone, or in this case in fact any other boned either. Unlike most land invertebrates such as beetles, they also lack a hard outer shell to protect them, but they re far from helpless however as they do have thousands of barbed stingers all along their tentacles, which can keep them safe from most predators.
Their stings are called nematocysts and the toxins they use are primarily to paralyze their prey so that the tentacles can pull it towards the mouth of the creature without it struggling and risk damage to their soft, fragile bodies. Although they are usually only poisonous enough to paralyze smell animals and fish, there are a few species that have been known to be fatal to humans, especially if they sting you more then once as they can cause paralysis, (which in water means drowning) or even cardiac arrest in some species such as the box jellyfish or the sea wasp.
Because of this extreme caution should always be exercised when you are swimming near jellyfish under any circumstances, unless you know for definite that they are totally harmless. There are also times when certain conditions cause jellyfish blooms, whereby thousands of them at once are present in small stretches of water, making no contact impossible, these should of course be avoided in general as they will often attract larger predators as well. Strangely their main predators are actually sea turtles, who are immune to their stings, largely due to having tough leathery skin that their barbs cannot penetrate.
They are certainly unusual and fascinating creatures, but can be dangerous so should always be approached with caution. Although true jelly fish actually don't have brains and cannot actively attack you because they cannot control their horizontal movements when at the surface of the water, if they drift into you by accident they will sting you if you touch their tentacles, and becoming entangled in them can be extremely dangerous.