Vitamins
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Vitamins are organic nutrients the body needs in small amounts to support normal growth, metabolism, and maintenance. They do not provide calories, and they are not interchangeable: vitamin A is a different substance with different jobs and dose limits from vitamin C or B12. Most vitamins must come from food because the body either cannot make them or cannot make enough. Vitamin D is a partial exception because skin can make it after ultraviolet exposure, but the season, skin exposure, age, and other factors affect how much vitamin D the body makes on its own. A vitamin deficiency can cause a recognizable problem, yet that does not mean taking more than you need will improve health or general wellness.
Sources: NIH Office of Dietary Supplements vitamin and mineral fact sheets; FDA dietary supplement overview.