Sharks are fish, but they belong to the group called Chondrichthyes, or cartilaginous fish, which is a different scientific group than most types of fish. Common fish, such as tuna or salmon, all belong to the group Osteichthyes. The distinction is based upon the makeup of their skeletons.
Sharks have skeletons made up of cartilage, a firm, flexible and hardy material that makes up the external ear and joints of human beings. All animals with backbones start out their embryological development with skeletons made of cartilage. In Osteichthyes, the bony fish, and in all other vertebrate groups, this cartilage is converted to bone, but in sharks and related fish, the cartilage remains. Although sharks are not bony fish, they share numerous anatomical features with bony fish, including jaws, brains, gills and fins.