One of a number of organic compounds required by living organisms in relatively small amounts to maintain normal health. There are some 14 generally recognized major vitamins: the water-soluble vitamin B complex (containing 9) and vitamin C and the fat-soluble vitamin A, vitamin D, vitamin E, and vitamin K. Most B vitamins and vitamin C occur in plants, animals, and microorganisms; they function typically as coenzymes. Vitamins A, D, E, and K occur only in animals, especially vertebrates, and perform a variety of metabolic roles. Animals are unable to manufacture many vitamins themselves and must have adequate amounts in the diet. Foods may contain vitamin precursors (called provitamins) that are chemically changed to the actual vitamin on entering the body. Many vitamins are destroyed by light and heat, e.g. during cooking.

http://lpi.oregonstate.edu/mic/vitamins Authoritative descriptions of all vitamins, plus other micronutrients, from the Linus Pauling Institute at Oregon State University