Arthritis is inflammation of the joint, which often causes pain and stiffness that can worsen with age. The most common types of arthritis are osteoarthritis and rheumatoid arthritis. Osteoarthritis causes cartilage—the hard tissue that covers the ends of bones where they form a joint—to break down. Rheumatoid arthritis is an autoimmune disorder that targets the lining of joints. Uric acid crystals, infections or underlying disease, such as psoriasis or lupus, can cause other types of arthritis.