They're a type of fish. Scientists have found over 340 different species of living Sharks, and fossils that show sharks have been swimming around the world's oceans for 430 million years.
They are of course best known for attacking and eating humans, but in reality most sharks are harmless to man.
What happens is that every year there are a small number of horrific attacks on swimmers and surfers. These get a lot of coverage in the media, that means when people hear the word shark they think - danger.