Most of the joints in the human body are designed to allow different bones to move, relative to each other (for example, the knee allows the fibula and tibia of the lower leg to move, relative to the femur of the upper leg). Where the bones meet (to form the joint), they are covered in a layer of smooth cartilage, designed to lower friction and reduce shock; in addition to this, most joints are lubricated by synovial fluid (similar in appearance to uncooked egg white). Osteoarthritis refers to a condition where the cartilage, and/or fluid, is damaged by wear and tear.

Osteoarthritis is a degenerative condition; that is, one that generally happens as a result of aging (it is not the same as rheumatoid arthritis, which is an auto-immune disease, a disease where the immune system actually attacks the joints in the body).