Perhaps the most familiar parts of a fish are its fins. Fins are not much more than sheets of skin that hang from the fish and are supported by a structure of rays to offer a fish stability when jetting through the water. These rays are made from organic material that can be soft or hard. Throughout evolution, some fins have evolved into protective devices like spines, or into a bait decoy that lures the prey closer to a hungry mouth.
To learn more, read The External Anatomy of a Fish