We've just completed a series on wines and body. We've addressed light, medium and full bodied white wines and red wines. And within this series, factors such as mouth feel, alcohol, tannin and residual sugar have been discussed. Fruit flavors also vary with lighter bodied red wines having red fruit flavors (e.g., cherry, raspberry) and full bodied reds having black fruit flavors (e.g., plum, black cherry, black currant, blackberry). And let's not forget about tannin. Light bodied red wines will have little or no tannin from the grape skins, seeds, stems and oak barrels, while full bodied reds can have big "chewy" tannin that can make you pucker.
One of the analogies that gets widespread use when discussing a wine's body is milk. The analogy states that light bodied wines are like skim milk, medium bodied wines are like 2% milk and full bodied wines are like whole milk. This is meant to address the characteristic of mouth feel. Skim milk is thin and watery while whole milk is thick and creamy. The milk analogy may be okay for comparing watery versus creamy, but it really doesn't work for wine.
Milk has fat solids that give it the mouth feel of creaminess. Wine doesn't. So maybe a better comparison of a wine's body and mouth feel might be with another common drink. And, since it works so well, let's take grape as an example. A grape drink that is made by mixing powder and water (e.g., Kool-Aid or Crystal Light) will generally be light bodied with more subtle flavors. Compare the powdered grape drink with pure grape juice (e.g., Welch's 100% Grape Juice) and then you get a better understanding of body from light to full. In this example, ignore the sweetness and just focus on how it feels in your mouth and the boldness of the fruit flavor.
Now, I think that's a better example of mouth feel that translates directly with wine. It's not perfect, but hopefully you get it. The way something tastes and feels in your mouth is very subjective and difficult to describe. Yet in the wine world, a significant amount of time is spent trying to describe flavors and mouth feel.
The best way to learn about a wine's body is through direct experience. Get a bottle of Pinot Noir and a bottle of Cabernet Sauvignon. Try tasting the Pinot Noir first, and then try the Cabernet. You should immediately experience light body versus full body. Cheers!