Grapes are composed of:

The stems comprise:

The inclusion of stalks can lead to higher levels of bitter tannin and potassium. This is more of a problem in red wines where the stems are present during fermentation, so grapes are often de-stemmed before crushing. In machine-harvested grapes, the stems normally remain on the vines.

The skin

The berry skin (epidermis) is covered by a thin waxy layer called the cuticle. This provides a waterproof coating and protection against injury. It contains fatty acids and sterols that stimulate yeast growth.
The pulp layers immediately beneath the epidermis contain a high proportion of the coloured pigments, tannins and flavour constituents found in grapes. Most quality winemaking varieties have small berries that give a high skin to pulp ratio.

The pulp

The pulp is the soft tissue of the grape, between the skin and the seeds. It is made up of large cells with very thin cell walls that have large spaces (vacuoles) containing the juice.
Grape juice in most cases is pale in colour. It is extracted from the berries before fermentation in white wine production. In red wine production the skins, pulp and juice are fermented together in order to extract the coloured pigments and tannins in the pulp cells underneath the skin.

The seeds

Seeds contain up to 50% of the phenolics of the grape, mostly tannins, some of which are extracted during the fermentation in red winemaking.

The image above is reproduced with the permission of the publisher, Winetitles Pty Ltd from Viticulture Volume 2: Practices, BG Coombe & PR Dry editors, 2001.