The short answer is: wine is grape juice that has been processed into alcohol by yeasts and made better with age. Though, to fully understand what a wine is you need to know how wine is made.
How Wine is Made
The essence of making wine is quite simple. In practice, however, this process can be taken to amazing depths of art and science to create the very best wines. Here is a basic overview of how wines are made.
Starting with the grapes, these are broken open and the stems are removed. The breaking of the grape allows the "free run" juice to pour forth. At this point the grapes have not been crushed, only broken open.
For red wines, the free run juice and broken grapes (called must) is then fermented in a tank with yeast. The yeast eats the sugar in the grape juice and releases alcohol and carbon dioxide as its byproducts.
Fermentation is complete when either the sugar has been completely consumed and the yeast can no longer feed or when the alcohol level is so high that it cannot survive. Winemakers want the yeast to consume all of the sugar. When this doesn't happen the fermentation is said to be "stuck" and further action is required to consume the remaining sugar.
For white wines the juice is separated from the must and fermented without the skins. Otherwise the process is the same.
Photo by: Mark Surman
Long ago wine was made with yeast that naturally occurs on the grapes. This was tricky because naturally occuring yeast may not be strong enough to complete fermentation. An incomplete fermentation results in a bad wine. Today specially cultivated yeasts can consume all of the sugar in the juice and produce wines with alcohol contents as high as 29%.
Once fermentation is complete and the yeast has consumed all of the sugar we now have wine. Although the flavor of this wine may not be very good as it is so young. At this point the broken grapes in a red wine will be crushed to extract more flavor and juice from the must.
To improve the flavor wine is aged. Most red wines and Chardonnay are aged in oak barrels. The rest is aged in steel tanks.
Oak adds many kinds of flavors to the wine including oak, spice, and vanilla. However, what the oak barrels also do is expose the wine to very minute levels of oxygen. This micro-oxygenation process helps develop the harsh young wine into a more enjoyable beverage over time.
Steel tanks do not impart any flavor on the wine. The aging process of so called "un-oaked" wines (aged in steel) is much shorter than that of oaked wines. When aging in steel tanks winemakers must provide a means of micro-oxygenation as the tanks do not breath on their own.
After six months to three years in the barrel the wine is taken from the barrels and bottled. Once in the bottle wines may be aged again anywhere from a few months to decades.