About sharks
Sharks are one of the world's oldest animals and have been around for millions(!) of years. Having basically completed evolution, sharks haven't changed much at all in that time, either.
Your safety around sharks
You are very unlikely to get bit by a shark. Sharks do not want to eat people, and you are more likely to get killed by a deer, vending machine, or chair than a shark.
Sharks can make mistakes, though, which is why you still want to respect a shark's space. However, some sharks are docile enough that you can swim with them. There's even cases where some divers pet sharks and find that the sharks keep coming back for more and more pats.
Sharks' safety around us
We are actually doing a lot more damage to sharks than they are to us. Hunted mostly for their fins, which are eaten in shark fin soup in East Asia, many sharks are killed everyday. It's gotten so extreme that sharks are actually becoming endangered. Sharks cannot breathe without swimming, and so when fishermen cut off their fins while they're still alive and drop them back into the ocean, sharks suffocate and die. This is not a humane or respectful way to end any animal's life. Yao Ming, the basketball player, has been working to decrease shark killings.