The phenols compounds in wine include a large group of several hundred chemical compounds, known as polyphenolics, which affect the colour, taste and mouthfeel of wine.
This large group can be broadly separated into two categories-flavonoids and non-flavonoids. Flavonoids include anthocyanins and tannins which contribute to the colour, taste and mouthfeel of the wine. Non-flavonoids include stilbenes such as resveratrol and compounds derived from acids in wine like benzoic, caffeic and cinnamic acid. In wine grapes, phenolics are found widely in the skin, stems and seeds.
In winemaking, the process of maceration or 'skin contact' is used to increase the influence of phenols in wine.

 

Phenolic acids are found in the pulp or juice of the wine and can be commonly found in white wines which usually doesn't go through a maceration period. The process of oak aging can also introduce phenolic compounds to wine, most notably in the form of vanillin which adds vanilla aroma to wines.
In red wine, up to 90% of the wine's phenolic content fall under the classification of flavonoids. These phenols, mainly derived from the stems, seeds and skins are often leeched out of the grape during the maceration period of winemaking. The amount of phenols leeched out during this process is known as extraction. In white wines the number of flavonoids is reduced due to less skin contact that they receive in winemaking. Like other flavonoids, the concentration of flavonols in the grape berries increases as they exposed to sunlight.
While phenolics comprise only 1% to 5% of wine constituents (majority water and alcohol), they are important because of their contribution towards appearance (colour), taste or mouthfeel (bitterness and astringency), and potential human health benefits.
Precipitated, they form an important part of a wines sediment and play a considerable role in the ageing of a wine. Red wines are much higher in phenolics than white wines, which is why red wine is better at protecting against heart disease.