The answer is in the question. Vitamin deficiencies are exactly that. There is a range of vitamins given letters such as A, C, D, etc. Some, eg B, exist as complexes which are groups, and these are designated as B1, B6 etc. Vitamins are necessary for normal biological functions, and deficiences of vitamins can lead to disease. For example, lack of Vitamin C can lead to scurvy and Vitamin D to ricketts. Minerals are also necessary for normal biological function. Some are obvious, such as iron for red blood cells and the like, but some are less obvious, such as selenium and vanadium. These are required in very small quantities on a regular basis from our diet, and that is why a varied diet is important for good health.