Arthritis is inflammation and tenderness of one or more joints. The main symptoms of arthritis are pain and stiffness in the joints, which often gets worse with age. The most common types of arthritis are osteoarthritis and rheumatoid arthritis. Osteoarthritis causes the breakdown of cartilage, the hard, slippery tissue that covers the ends of bones where they form a joint. Rheumatoid arthritis is a disease in which the immune system attacks the joints, starting with the synovial membrane.