Grown on a vineyard and crushed/fermented in a wine barrel, these are used to make wine.
A complex item, they appear in inventory as "grapes:<vine type>-<harvest id>". <vine type> is dependent on the vine cutting used (see the Viticulture page for some), <harvest-id> is dependent on the player (each time you harvest a vine, this number will increase by one). An example might be: "grapes:Dawn's Blush-53" (53rd vine harvested by that player).
See the Wine pages for more info.