An endoskeleton is the skeletal structure possessed by all human beings and by many animals; it is composed of a support scaffold made of mineralized tissue forming either bone or cartilage. All vertebrates possess these support structures.
Animals like sponges possess complex endoskeletons, though their only function is supporting the mass of the sponge and providing shape to its orifices. In more complicated animals, the endoskeleton serves as an anchor for muscle structures. In a crocodile or human being these muscles are anchored to the bones and interact with them to flex, produce force, and accomplish all daily tasks necessary to ensure the health and survival of the organism.
Endoskeletons are derived from mesodermal tissue and during the development of the organism in utero, or in the egg, they are composed largely of cartilage while the place of the vertebral column is taken by a notochord. These substances are almost always repurposed during development, leading to the formation of bone mass in the animal by the time of birth.
Some animals, like sharks, develop very few bones and have endoskeletons composed largely of cartilage. They live their entire adult lives with cartilaginous supports which leave no fossil record. These endoskeletons are generally more flexible than bone but are less strong in the resistant sense.