Arthritis is an inflammation of the joints. It can affect one joint or multiple joints. There are more than 100 different types of arthritis, with different causes and treatment methods. Two of the most common types are osteoarthritis (OA) and rheumatoid arthritis (RA). The symptoms of arthritis usually develop over time, but they may also appear suddenly. Arthritis is most commonly seen in adults over the age of 65, but it can also develop in children, teens, and younger adults. Arthritis is more common in women than men and in people who are overweight.