The joint where your upper arm connects to your body is a ball-and-socket joint. The bone in your upper arm, called the humerus, has a round end that fits into the curved structure on the outside of your shoulder blade.
Ligaments and tendons hold it together. Ligaments connect the bones, while tendons connect muscles to the bone. A layer of tissue called cartilage keeps the bones apart, so they don't rub against each other. The ball and socket lets you move your arm up and down, back and forward, or in a circle..