Arthritis most often affects areas in or around joints. Joints are parts of the body where bones meet, such as your knee. The ends of the bones are covered by cartilage, a spongy material that acts as a shock absorber to keep bones from rubbing together. The joint is enclosed in a capsule called the synovium. The synovium's lining releases a slippery fluid that helps the joint move smoothly and easily. Muscles and tendons support the joint and help you move. Different types of arthritis can affect one or more parts of a joint. This frequently results in a change of shape and alignment in the joints.

Certain types of arthritis can also affect other parts of the body, such as the skin and internal organs. There are more than 100 different types of arthritis. It is important to know which type of arthritis you have, so you can treat it properly. If you don't know which type you have, consult with a doctor. Some common types of arthritis are described below.