Arthritis is a joint disorder causing inflammation in one or more joints. It is common in adults 65 and older, but it can affect people of all ages. Arthritis is used to describe a range of different diseases and conditions that affect joints, the tissues that surround joints, and other connective tissue. The most common form of the condition is osteoarthritis which occurs when the cartilage lining the bones in the joint breaks down due to wear and tear.