What Is Arthritis? Arthritis is inflammation of one or more of your joints. The main symptoms of arthritis are joint pain and stiffness, which typically worsen with age. The most common types of arthritis are osteoarthritis and rheumatoid arthritis. Arthritis by the Numbers: Nearly 53 million adults have doctor-diagnosed arthritis; that number is expected to grow to 67 million by 2030. Almost 300,000 babies, kids and teens have arthritis or a rheumatic condition. Arthritis is