From NuGOwiki

Vitamins are nutrients required in very small amounts for essential metabolic reactions in the body. The term vitamin does not include other essential nutrients such as dietary minerals, essential fatty acids, or essential amino acids. Nor does the term refer to the large number of other nutrients that promote health, but are not strictly essential. Vitamins can be subdivided in fat-soluble and water-soluble vitamins. Vitamins are biomolecules that act both as catalysts and substrates in chemical reactions. When acting as a catalyst, vitamins are bound to enzymes and are called cofactors, for example vitamin K forms part of the proteases involved in blood clotting. Vitamins also act as coenzymes to carry chemical groups between enzymes, for example folic acid carries various forms of carbon groups (methyl, formyl or methylene) in the cell.