Cartilage is a type of tissue made up almost entirely of the protein collagen, which is arranged in a weave much like the construction of fiberglass. Collagen is the most common protein in the body and makes up the majority of the body's proteins, and is the primary material comprising bones, skin, ligaments, and tendons. However, in cartilage, collagen is unsupported by other materials, which gives it a consistency that is stiffer than tendons and ligaments, but more flexible than bone. The skeleton and skin of sharks is made entirely of cartilage.
Cartilage is found primarily on the surfaces of bones where they meet with other bones, in the discs that separate the vertebrae in the spine, in the sternum that supports most of the ribs, as well as forming most of the shape of the nose and ears. Its primary purpose is allowing bones to move against each other without grinding against each other. A lack of cartilage between articulating bones leads to osteoarthritis. A lack of cartilage between the vertebrae in the spine can lead to functional paralysis and the need to fuse vertebrae.
Cartilage is not a living tissue and, unlike bones and skin, contains no blood vessels. It accumulates on the end of structures in a very slow manner. As such, when it is damaged, it's very slow to repair itself and, if there is a discontinuity in the cartilage producing tissue, it may not regrow at all.