A dietary supplement is either intended to provide nutrients in order to increase the quantity of their consumption, or to provide non-nutrient chemicals which are claimed to have a biologically beneficial effect.
A vitamin is an organic compound and an essential nutrient that an organism requires in limited amounts. An organic chemical compound (or related set of compounds) is called a vitamin when the organism cannot synthesize the compound in sufficient quantities, and it must be obtained through the diet; thus, the term vitamin is conditional upon the circumstances and the particular organism.
For example, ascorbic acid (one form of vitamin C) is a vitamin for humans, but not for most other animals. Vitamin D is essential only for people who do not have adequate skin exposure to sunlight, as ultraviolet light promotes synthesis in skin cells.
Supplementation is important for the treatment of certain health problems, but there is little evidence of nutritional benefit when used by otherwise healthy people. Other reasons for believing in supplements are for lack of energy, concentration, learning and sleeping.