Color and tannins.
Only six to ten cells thick, the skin is full of aromas, potassium and phenolic compounds such anthocyanins pigments responsible for the wine color and tannins responsible for the astringent and bitter taste. The skin is more important in red wines.
Water, sugar, acids.
The pulp beneath the skin contains the juice: sugar, water, aromas, potassium, acid (tartaric and malic). The pulp is more important in white wines where it gives flavor and acidity.
Seeds are full of tannins and winemakers should not crush them during the pressing.
Stems are full of tannins and winemakers should not crush them during the pressing.
Fructose, Glucose, Sucrose.
Grapes contain a combination of sugars. Fructose is the sweetest and the predominant when berries are overripe. Glucose and sucrose are less sweet. Not all sugars are fermentable by yeast, that´s why some varieties taste sweet even after fermentation to "dryness".
Tartaric, Malic, Lactic.
Tartaric and Malic acid are the primary ones. They vary with varietal, climate and ripeness. Tartaric acid tends to bind with potassium if stored too cool and is responsible for creating the "crystals" or "wine diamonds". Despite what people think, "wine diamonds" (tartrates) are not a sign of bad quality. Actually it can be seen as a sign of quality, indicating that the wine was not overprocessed. Wine crystals never impart an unpleasnt taste.
Lactic comes from latin "Lactis" and it means milk, in fact is Lactic acid found in milk and diary products, and gives wine a buttery, creamy taste.
Malic acid can convert to Lactic acid in presence of proper bacteria. This is called Malolactic Fermentation (MLF). This is an optional fermentation. Winemakers do not want to risk a spontaneous MLF occurring in the bottle with consequential gas release and corks blowing or bubbly wines (if the gas stays in the bottle).
Primary, Secondary, Tertiary.
Primary aromas are in the skin and in the grape varietal.
Secondary aromas come from fermentation where different yeasts produce different aromas. Some yeasts are known to be more fruity.
Tertiary aromas come from aging and maturation. The main factors are: the type of oak, toasting levels and bottle maturation.
Aromas and flavors vary also with climate, varietal, ripeness level at harvest, age and struggle of the vines.