Tannins are formed upon the decomposition of vegetation. These compounds are large molecular weight organics that fall under two large primary categories: Humic acids and Fulvic acids. These are simplified structures, but they show the complexity of tannins.
Tannins can cause a yellow to brown cast in water and may also impart a taste and odour. They will generally be found in surface water supplies or shallow wells. Although these compounds are not a health risk, they are aesthetically displeasing.