The skeletal system consists of bones, cartilage, and connective tissue that hold bones together. Bones have two types of tissue - compact bone that is solid and spongy bone that has many open spaces. Bones perform four key functions: providing structure, allowing movement, protecting organs, and storing minerals. There are three types of joints - fixed, slightly movable, and freely movable. Common injuries to the skeletal system include fractures and broken bones, while diseases include arthritis which causes joint swelling and stiffness and osteoporosis which reduces bone density.