Vitamin D, also known as calciferol, is actually two fat-soluble vitamins including vitamin D2 and vitamin D3. The term vitamin D includes both of these when referred to. Vitamin D3 is unique in that the body produces it when the skin is exposed to ultraviolet B radiation, or sunlight.Vitamin D is important in the maintenance of the organ systems. It regulates the calcium and phosphorous levels in the blood and promotes the absorption of the nutrients from your food in the intestines. It also promotes the re-absorption of calcium in the kidneys. Perhaps the most important function of vitamin D is that it promotes bone formation and mineralization. This is essential in developing and maintaining strong bones.