The skeletal system has five main functions: providing movement through muscle attachment points on bones, supporting the body, protecting organs, making blood cells, and storing minerals. It consists of two main parts - the axial skeleton including the skull and spine, and the appendicular skeleton including the shoulders, arms, hips and legs. Bones come in four basic shapes and have a layered structure of periosteum, compact bone, spongy bone, and bone marrow. Joints allow bones to move and come in both immovable and movable types, including ball-and-socket, hinge, pivot, and gliding joints.