Vitamins are necessary nutrients to allow your body to function normally.  Insufficient intake of certain vitamins can lead to vitamin deficiency which can in turn lead to illnesses and diseases.  Since your body cannot produce vitamins on its own, you have to either get them from the foods you eat or nutritional supplements.  

Most vitamins can be taken in significantly higher doses than the recommended dosage without adverse reactions.  However, fat soluble vitamins may be toxic at high doses because they remain in your body for a long time.  In particular, overloading on vitamin A or D is dangerous.  Megadoses or even very high doses of water soluble vitamins, such as vitamin C, may be toxic or dangerous to some people.  Reducing the dosage usually fixes the problem.

There are 13 known vitamins and they are categorized as fat or water soluble vitamins.

Fat soluble vitamins:  Vitamins A, D, E and K.  Your body stores these vitamins for relatively long periods (months or even years).

Water soluble vitamins:  eight B vitamins and vitamin C;  these remain in your body only for short periods and must be replenished more often (except for vitamin B12).