Water-soluble vitamins are those soluble in water. Fat-soluble vitamins are those soluble in oil (lipids, fat).Vitamin C and the vitamins of the B complex are examples of water-soluble vitamins. Vitamins A, D, E and K are examples of fat-soluble vitamins.
Fat-soluble vitamins, since they are not soluble in water, cannot easily be excreted by the body. So they tend to accumulate in tissues with toxic effect when they are ingested in amounts over what is necessary.