It depends on the type of shark as various species have different habits.
Most sharks are meat eaters and eat fish, large seals, and even other sharks! Some, including the whale shark, are filter feeders. They sieve small aquatic life including mollusks, clams and plankton. Larger sharks aren’t afraid to go after a huge meal such as a dolphin or a sea lion. Sharks also eat crab, squid, and lobster in areas where it is available.
Tiger sharks are probably the ones with the most ravenous appetite. These scavengers of the sea are known to swallow inedible objects such as chains or license plates.