What exactly is a wine's "structure"?

Hello there! I'm Dr. Vinifera, but you can call me Vinny. Ask me your toughest wine questions, from the fine points of etiquette to the science of winemaking. And don't worry, I'm no wine snob—you can also ask me those "dumb questions" you're too embarrased to ask your wine geek friends! I hope you find my answers educational, empowering and even amusing. And don't forget to check out my most asked questions and my full archives for all my Q&A classics.
Dear Dr. Vinny,
My question is about the term “structure.” I know that those components present in the wine have certain structure, but what exactly does it mean when we are talking about the wine itself?
—Osvaldo G., Calgary, Alberta
Dear Osvaldo,
You’re right that “structure” refers to the relationship of different components in wine, such as acid, tannin, alcohol and glycerol. It’s one thing to describe the body of a wine (how it feels in the mouth, like the difference between skim milk and heavy cream, for example), or how the acidity might make your mouth water. But it’s the relationships between these elements that together make up a wine’s structure.
Structure itself doesn’t describe the flavor, but it’ll give you some clue as to how the flavors of a wine will age: wines with good structure are more likely to age well, while wines lacking in structure are unlikely to improve in the cellar.
—Dr. Vinny