Collagen is a main structural protein in the body. It is the main component in many of the connective tissues in the body. In fact it is the most abundant protein in mammals.
Collagen can be found in tendons, ligaments, cartilage and many other areas.
Collagen can be harvested and used in other ways. It can be used to make sausage casings and as string for musical instrument.
Collagen can be transplanted as well. Just like a liver or kidney transplant, certain parts made of collagen can be reused. It can be used in cosmetic surgery to repair damaged parts.
There are a lot of companies coming up with substitutes for collagen. In the case of sausage casings, there is a very adequate replacement that is more often used now.