A shark's only real weapon is its mouth. Like its fins and sense organs, the shark's mouth is a highly effective physiological adaptation, perfectly suited to its job. Sharks terrify people because some of them can, and sometimes will, feed on human beings.
Sharks do attack humans from time to time, but the risk of attack is actually very slight.
Only a small proportion of shark species have been known to attack, and even these species exhibit this behavior very rarely. These sharks typically attack defensively, when a person has wandered into their environment, or accidentally, because they've mistaken a person for their standard prey. In both cases, chances are the shark will flee after only one bite. They don't have any real interest in humans as a food source. Between 75 and 100 shark attacks are reported every year, and fewer than 20 are fatal.