Osteoarthritis or degenerative joint disease is the most common type of arthritis and is seen especially among older people. Osteoarthritis mostly affects cartilage, the hard but slippery tissue that covers the ends of bones where they meet to form a joint. In osteoarthritis, the surface layer of cartilage breaks and wears away. This allows bones under the cartilage to rub together, causing pain, swelling, and loss of motion of the joint.
Click here for more information. http://www.niams.nih.gov/Health_Info/Osteoarthritis/default.asp