Vitamins are carbon-comprised organic chemical compounds the body needs to consume for growth, repair, metabolism and overall health. Exceptions are vitamins D, K, choline and biotin, which technically the body can self-produce.
Vitamins, along with minerals, help make the enzymes and hormones necessary for all the wondrous chemical reactions we need to live, including manufacturing blood cells and converting food into usable energy. Some are needed in smaller amounts than others, but all are equally important.
Vitamins are found in plant and animal food sources, but are not foods in and of themselves. If your diet alone is not providing you with an adequate measure of all these vital nutrients, supplements can be beneficial.
You will notice that some vitamins have one or two other names associated with it, and some of the B's are referred to by not-necessarily-sequential numbers. This is not scientists' attempt to confuse us, but simply a product of an evolving understanding of these essential elements. Some B vitamins were discovered and assigned a number only to be later found to be two separate elements, and so the numbering/naming system got a little out of order.
Some vitamins became more commonly referred to by their full names, others by their letter and others by their letter and number combo.