Vitamins are compounds which come in two varieties, fat soluble and water soluble. Vitamins are essential micro-nutrients which assist in many functions of metabolism.
While vitamins are not used as building blocks or energy, they are still essential because they help assist the nutrients that do perform those functions. For example, vitamin C helps improve iron absorption; vitamin K is crucial to blood clotting, so you don’t bleed to death when you cut yourself while making dinner for that significant other.
There are many vitamins which come in the form of supplements and pills. However, most of the time these vitamins can just as easily be found naturally in food. Generally speaking, if you eat a wide variety of fruits and vegetables, you will adequately meet the RDI (recommended daily intake) for all vitamin with some rare exceptions such as B12 where a supplement or functional food may be required.
A simple principle to apply to ensure you are getting all your vitamins is to eat the rainbow. Make sure you are eating a wide variety of fruits and vegetables that are of different colours. There is a reason a strawberry is a different colour to an orange they have different nutrients.