There are hundreds of shark species in the world, all of which have slightly different eating habits. An individual description of the diets of each family of sharks would be impossible, but there are some general consistencies throughout. For instance, all sharks eat meat to some degree. Sharks are to the oceans what lions are to the African plains, meaning that they are at the top of their respective food chain. They often weed out the sick and weak animals and contribute to the "survival of the fittest" in their realms. Perhaps ironically, the largest of all sharks, known as the whale shark, feeds almost exclusively on small fish and plankton.
Popular culture has tended to cast sharks in the role of dangerous, belligerent animals who attack humans at the slightest provocation. While it is true that some species of sharks go after large prey, like seals and other sea mammals, they only threaten humans when they feel threatened themselves. Most sharks eat a little bit of everything that they can find in their local area. This is not just because that is the most convenient option, but also because sharks usually only eat once every two to three days, and when they do eat, it can be as much as three percent of their total body weight. Almost everything in the sea is eaten by some type of shark.
The types of food that generally appeal to the most sharks are things like squid, mid-sized fish, and some types of crustaceans. Some sharks have been known to swallow non-food items that make their way into the oceans from land, such as license plates and other man-made objects. Tiger sharks are the best example of this type of eating. These sharks eat basically anything in their path, including large sea birds and turtles, and have developed a reputation of being opportunistic and greedy eaters because of this.
To make possible the wide variation of things that sharks eat, they have evolved many different feeding mechanisms. In species that consume primarily meat, many rows of sharp teeth rip and tear their food, and are constantly being replaced as they fall out. Sharks do not chew well, however, and proper digestion can take some time. This is one reason why their meals are as infrequent as they are. Some species of sharks eat primarily plankton and smaller marine life, and so their jaw structure is more loosely attached and oddly shaped, to enable them to vacuum up their prey from the sea floor, or obtain food in other unusual ways.