Vitamins are organic compounds required for daily functioning and health; most are obtained largely through the diet. There are a few exceptions: vitamins A, B3 (niacin), and D can be produced within the body and vitamins K and B7 (biotin) are produced by bacteria in the intestinal tract. Minerals are inorganic compounds, so they are not produced by living things and are found in soil and water.
Both vitamins and minerals are essential for proper functioning of the body, so it seems natural to take a daily multivitamin to ensure full intake. Since vitamins and minerals come from the diet, a multivitamin may be a good idea if you consistently eat a poor quality diet and cannot modify this. For example, a strict vegan needs to supplement with at least vitamin B12 (cobalamin). For a person with a diet of even moderate quality and diversity, however, large studies have not found any health benefits to multivitamin supplementation. In some cases, there may even be some slight health risks to overdoing supplementation!
The take home is clear: eat a varied diet and you don’t need to worry about multivitamins.