An endoskeleton is an internal skeletal structure that provides support, protection, and shape to an organism. It is made up of bones or cartilage and is found in vertebrates, including mammals, birds, reptiles, amphibians, and fish. Unlike exoskeletons, which are external structures seen in insects and crustaceans, endoskeletons grow with the organism and allow for greater mobility and flexibility. They also serve as attachment points for muscles, facilitating movement.