Arthritis is a general term for a group of more than 100 diseases. The word “arthritis” means “joint inflammation.” Inflammation is one of the body’s natural reactions to disease or injury, and includes swelling, pain, and stiffness. Inflammation that lasts for a very long time or recurs, as in arthritis, can lead to tissue damage.

A joint is where two or more bones come together, such as the hip or knee. The bones of a joint are covered with a smooth, spongy material called cartilage, which cushions the bones and allows the joint to move without pain. The joint is lined by the synovium. The synovium’s lining produces a slippery fluid — called synovial fluid — that nourishes the joint and helps limit friction within. External to it is a strong fibrous casing called the joint capsule. Strong bands of tissue, called ligaments, connect the bones and help keep the joint stable. Muscles and tendons also support the joints and enable you to move.

With arthritis, an area in or around a joint becomes inflamed, causing pain, stiffness and, sometimes, difficulty moving. Some types of arthritis also affect other parts of the body, such as the skin and internal organs.