A joint is where two or more bones come together, like the knee, hip, elbow, or shoulder. Joints can be damaged by many types of injuries or diseases, including

• Arthritis - inflammation of a joint. It causes pain, stiffness, and swelling. Over time, the joint can become severely damaged.
• Bursitis - inflammation of a fluid-filled sac that cushions the joint
• Dislocations - injuries that force the ends of the bones out of position

Treatment of joint problems depends on the cause. If you have a sports injury, treatment often begins with the RICE (Rest, Ice, Compression, and Elevation) method to relieve pain, reduce swelling, and speed healing. Other possible treatments include pain relievers, keeping the injured area from moving, rehabilitation, and sometimes surgery. For arthritis, injuries, or other diseases, you may need joint replacement surgery to remove the damaged joint and put in a new one.

NIH: National Institute of Arthritis and Musculoskeletal and Skin Diseases