Wine Components

There are main elements to every wine. This is where all the different smells and flavors start. This is also a good place to begin if you're just learning how to describe wines. This knowledge is useful in describing what sort of wine you're looking for when in a store or restaurant.

The main components

Alcohol

Missing one of the components
so is it really wine?

Alcohol adds flavor and texture/viscosity. If overdone it can give a sensation of sweetness or burning  -- this is something you should not pick up in a table wine.  Generally, cooler climate grapes give lower alcohol wines, warmer climates give higher alcohol wines. It's also a stylistic choice, as most New World wines are generally higher alcohol than the traditional Old World wines.

Tannin

That dry feeling in your mouth from some red wines. It's the same sensation as black tea because it also has tannins. Makes a wine more complex, but can be bitter. White wines are lower in tannins than reds. Barbera, Grenache, and Pinot Noir are lower-tannin reds. Cabernet Sauvignon and Syrah are higher-tannin red wines.

Acidity

If done right it's a mouthwatering sensation. Think of a refreshing Sauvignon Blanc. Or even think of Coca-Cola which is very high in acid and sugar. A few people will pick up a bitter or even sour sensation from higher acid wines. Sauvignon Blanc and sparkling wines are higher in acid. Grenache, Merlot, and Chardonnay are lower-acid wines. These might be what you're looking for if you suffer from acid reflux.

Sweetness 

Residual sugar is the fructose and glucose left in a wine after fermentation. Dessert wines are sweet by design. Sparkling wines have different amounts of added sugar. Some table wines also have sugar purposely left in to "enhance" the flavor, but good luck getting the winery to admit it. This is very common in cheap wines, but you'll find residual sugar in some $100+ Cabernets, too!

This is different from fruitiness, which is subjective, so your brain will see it as the same. Also, high alcohol can be sensed as sweetness. It can get a little tricky separating residual sugar from a fruit forward wine or high alcohol. A fruit forward style of wine is more likely to be higher in alcohol and maybe even have some residual sugar.  

Yes, determining sweetness in a wine can be tricky.

Gives you

Body

Sugar, alcohol, and tannins determine the weight of the wine. A light-bodied wine means it's low in alcohol and tannins. A full-bodied red wine will be higher in any or all of the four components described above, though alcohol is usually the key. A fuller-bodied white is generally higher alcohol, generally 14% or so, and often aged in oak barrels. Think of those heavier, oily, oaky California Chardonnays.

Balance 

When the above elements are in harmony and no one of them stands out as too much or too little. This is subjective as you don't have to prefer wines that might be described as perfectly balanced. You might prefer lower tannins, a bit of sweetness, etc.

Structure

This is similar to balance in description in that you are looking at fruit, alcohol, acid, and tannin, but to help determine ageability. How is it different from balance then? You may find a young Cabernet, for instance, is not balanced currently, but has the structure to age well.

The poster child for a full-bodied chardonnay

Examples

A Chardonnay that's rich, oaky, and shows lots of texture, maybe a bit oily, is full-bodied. Unoaked Chards are lighter-bodied.

A lighter-bodied Pinot Noir means lower alcohol and tannins. If there's no sweetness and the acid is kept in check, it is a balanced wine, Something you might say has finesse or is delicate.

A young Cabernet with a decent amount of fruit, not too much alcohol, with fairly high acid and tannin is something you could say has the structure to age well.