There are nearly 100 different types of arthritis, but physicians often refer to two main categories: osteoarthritis and rheumatoid arthritis. Osteoarthritis occurs when the cartilage – the surface that cushions the ends of the bones – wears away over time, and bone rubs against bone. Rheumatoid arthritis is an autoimmune disorder that affects the lining of the joints. Either condition can cause pain, stiffness, and difficulty moving the joints.