What is it?
Vitamin D is a group of fat-soluble prohormones, the two major forms of which are D2 (or ergocalciferol) and D3 (or cholecalciferol). The term vitamin D also refers to metabolites and other analogues of these substances. Vitamin D3 is produced in skin exposed to sunlight, specifically ultraviolet B radiation.
Much more than Vitamin D is discussed at the Natural Source Vitamins Homepage