“Shark” is a broad term referring to all sharks and their relatives, which are jawed fish with cartilaginous skeletons. There are no true bones present in a shark’s skeleton, fins or scales.
Scientifically known as Chondrichthyans (Greek for ‘cartilage fish’), sharks and their relatives are split into two subclasses: Elasmobranchii (sharks, rays and skates) and Holocephali (chimaeras). Elasmobranchs (Greek for ‘plate gills’) have placoid scales called dermal denticles, a true upper jaw and lower jaw with rows of teeth, and five to seven paired gill openings. Holocephalans (Greek for ‘whole head’) have three pairs of tooth plates, jaws fused to their craniums, and single pair of soft gill openings.
Combinations of other traits such as presence or absence of anal fin, presence or absence of spine, shape of caudal fin, dentition (teeth), and location of mouth split sharks and their relatives further into family, genus and eventually into species. The character you need to remember is a skeleton made of cartilage. Ultimately, that is what makes a shark a “shark.”