Cartilage Physiology: Overview
Articular cartilage, most notably that which is found in the knee joint, is most generally characterized by very low friction, high wear resistance and poor regenerative qualities. It is comprised of cells, called chondrocytes, and their surrounding extra-cellular matrix (ECM) which is comprised of proteins and water, among other things. It is responsible for much of the compression resistance and load bearing qualities of the knee joint and, without it, walking is painful to impossible.
Image Credit: http://www.cartilagecareclinic.com/CCInformation.htm
Above: Cartilage in normal and increasingly damaged states