Tannins are plant secondary metabolites widely synthesized and distributed in many plant species. They play a role in plant growth regulation and also in protection from predation due to their astringent character rendering plant tissues inedible. The word tannin has a long and well-established usage in the scientific literature. Indeed, tannins are divided into four groups with two majors: the condensed tannins or proanthocyanidins and the hydrolyzable tannins. This article will describe, in the first instance, the condensed tannins and then the hydrolyzable tannins. Descriptions will include their structures, biosynthesis, and occurrences, especially in wines.