Arthritis is a general term for inflammation in the joints. Arthritis can occur in numerous forms. The most common is osteoarthritis, a condition in which the cartilage, the protective cushioning between the joints, wears out.When this happens, the bones rub directly against other bones.This causes structural changes that can be seen on X-rays.he joints most often affected are the hands and the weight-bearing joints of the lower extremities (knees, hips, feet). In osteoarthritis, there is a steady worsening and decline of joint cartilage.