Vitamins are, by definition, compounds necessary in trace amounts for the normal functioning of the human body. Are they important? They are vital. We need vitamins in order to see the world around us, to grow, to make bones and connective tissue, to fight infections and cancer, to heal wounds, to stop from bleeding to death, and to keep our teeth from falling out.
We are not self-sufficient. We depend on a steady supply from outside sources for these vital compounds. Vitamins cannot be manufactured in sufficient amounts by the body, and must be taken in from the environment. They occur naturally in many foods (vitamin D is manufactured by the body in response to sunlight exposure -- 15 minutes a week is all that is needed)
Vitamins also assist in the formation of hormones, blood cells, nervous-system chemicals, and genetic material. The various vitamins are not chemically related, and most differ in their physiological actions. They generally act as catalysts, combining with proteins to create metabolically active enzymes that in turn produce hundreds of important chemical reactions throughout the body. Without vitamins, many of these reactions would slow down or cease. The intricate ways in which vitamins act on the body, however, are still far from clear.
A good multi is a good basis