Arthritis is a condition in which one or more joints enlarge or become inflamed. It is a condition that affects the joints. It also includes joint degeneration or inflammation. Bones sustain the body and facilitate movement. Joints, such as your elbow or knee, are sites where two bones meet. They might be immobile, slightly moveable, or move freely. The moveable joint is surrounded by a synovial membrane. Some types of arthritis can affect your organs, including your eyes and skin.