A vitamin is an organic compound required in tiny amounts for essential metabolic reactions in a living organism. The term vitamin does not include other essential nutrients such as dietary minerals, essential fatty acids, or essential amino acids, nor does it encompass the large number of other nutrients that promote health but that are not essential for life.
     You need only small amounts (that's why they are often referred to as micronutrients) because the body uses them without breaking them down, as happens to carbohydrates and other macronutrients. So far, 13 compounds have been classified as vitamins. Vitamins A, D, E, and K are the fat-soluble vitamins. Vitamin C and the eight B vitamins-biotin, folate, niacin, pantothenic acid, riboflavin, thiamin, vitamin B6, and vitamin B12 are dissolve in water.

Vitamins have various names:

Vitamin A = retinol, retinaldehyde, retinoic acid
Vitamin B1 = thiamin
Vitamin B2 = riboflavin
Vitamin B3 = niacin, nicotinic acid, niacinamide
Vitamin B5 = pantothenic acid
Vitamin B6 = pyridoxine, pyridoxal, pyridoxamine
Vitamin B9 = folic acid, folate
Vitamin B12 = cobalamin
Vitamin C = ascorbic acid
Vitamin D = calciferol
Vitamin E = tocopherol, tocotrienol
Vitamin K = phylloquinone
Vitamin H = biotin