Collagen is a simple protein. It is composed of 3 long chains wound together in a tight triple helix. Each chain is 1400 amino acids long. A repeated sequence of the amino acids glycine, proline and hyrdroxyproline make up each chain. There are 13 types of collagen, based on their formation. Some are long "ropes" while others are thin flat sheets.

In the Body:
Collagen accounts for about a quarter of the protein in your body, being a major structural protein. Collagen stregthens tendons, skin and the membranes that hold the internal organs in place. Collagen mixed with mineral crystals compose your teeth. Distribution and orientation of collagen directly relates to the function of the tissue. (For example collagen in tendons transmits tension). Collagen is integral to wound healing and blood clotting.

As a Treatment:
Urinary Incontinence is treated by injecting collagen into the urethra, where it forms a small bulk, decreasing the diameter of the urethra. This allows the patient to better control urination as the pressure against their muscles is reduced.
Bone Grafts can now be preformed using collagen and the patients bone marrow as opposed to bone (painfully) harvested from the patient. The mixture is used as a "scaffold" for the new bone to grow around/inside.
Vision correction can be achieved using a semi-permantent contact derived from collagen. It can correct far-sightedness and astigmatism.
Anti-Cancer therapy utilizes collagen as a mode of transport to get the anti-cancer agent to the tumor.
Wound healing is accelerated by applying this protein to the site of the wound. This is particularily important for chronic wound healing.

Cosmetic Surgery:
Collagen injections are used to decrease the appearance of wrinkles and furrows. They can also be used to help scars heal. The collagen (derived from cow hide) is injected just beneath the skin where the body accepts it as its own, and allows it to fill the wrinkle or heal the scar. Injections for anti-aging effects last up to two years. Many patients go in several times a year to have touch-up injections.

Collagen Induced Arthritis:
Collagen (type II, bovine) is used to induce arthritis in animal studies. These studies are preformed to evaluate anti-inflammatory treatments. Particular mice (a strain called DBA/1) which are particularily sensitive to the collagen are used in the studies.
This makes one wonder if the collagen used in cosmetic treatments induces, increases or hastens the development of arthritis of humans, even in some small way.

Scurvy is caused by a lack of vitamin C, which is required for the synthesis of hydroxyproline, a necessary component of collagen. Without collagen your body cannot repair itself normally, so tooth loss and bruising (among many other things) occur. This can lead to death.

In Cooking:
When collagen is heated the helix structure breaks down and the these unbound chains soak up any moisture available, up to 60 times its weight. The result is a gelatin.