What is Tannins in wine

Tannins are chemicals that occurs naturally. Tannins can be founds in tree bark or leaves and also founds in a wine ingredient such as  seeds and stalks of grapes and oak barrel that used to store wine. Each types of grape has different of tannins such as 

 Sauvignon, Cabernet, Petit Verdot and Nebbiolo has a highest Tannins of the grapes variety. Tannins act like the alcohol of wine. 

Tannins in a fresh red wine have an astringent and bitter taste. However, If the red wine is aged ready to drink, Tannins have a sweet taste. and the bitter taste will be lighter