Vitamins are an ingredient of organic amino acid molecules necessary to the proper creation and function of proteins in living organisms. Because proteins in all living things are slightly different, different vitamins are required depending on species.
When an organism does not get the vitamin it needs, it will suffer from disease triggered by lack of the protein involved. Many vital amino acids are created by the organism that needs them, but for almost every animal, there are some helpful amino acids that require nutrients not produced by the body or that are not produced in sufficient amounts. Vitamins help fill this deficiency.
Vitamins that are water-soluble need to be replenished daily; those that are fat soluble are typically necessary only in trace amounts because they are stored in the body's fat reserves, and often can be recycled for a very long time.
Not all vitamins are amino acids; many are metabolic precursors, or pieces of amino acid that are necessary to build specified amino acids. Vitamin A can be produced from beta carotene, for instance, and sufficient ultraviolet light makes plenty of vitamin D in humans.
Vitamins skip letters because many substances originally thought to be vitamins turned out to be something else. Necessary vitamins for humans include fat-solubles A, D, E, and K, and water-solubles B complex (8) and C.
Web Resources On Vitamins
Harvard School of Public Health: Vitamins
Vitamins and Coenzymes
Book Resources On Vitamins
The Doctor's Complete Guide to Vitamins and Minerals by Mary Dan Eades
Nutritional Biochemistry of the Vitamins by David A. Bender