Arthritis is a condition that causes inflammation and pain in one or more joints. It can affect people of all ages, but it's most common in older adults. There are several types of arthritis, including osteoarthritis (which occurs when cartilage wears down over time), rheumatoid arthritis (an autoimmune disorder that causes inflammation and pain in the joints), and psoriatic arthritis (a form of arthritis that affects people with psoriasis). Treatment for arthritis may include medications, physical therapy, or surgery.