Add a shot of vinegar to a simple stew of white beans and shrimp and notice how the seemingly simple, earthy flavor of the beans suddenly gains definition and complexity. Do the same thing with a soup of puréed winter squash and see how a dish that once was dominated by rich and sweet now has a round, full fruit character.
Though the results may be similar, salt and acidity work slightly differently. Salt is a flavor potentiator -- in other words, it works chemically to make other flavors taste more of themselves. Acidity works as seasoning by giving a dish backbone or structure, which allows other flavors to stand out and shine.
It doesn't take much. Just as with salt, you don't want to taste the seasoning itself; you just want the effect it has on other flavors. Sometimes only a couple of drops of lemon juice will be all that it takes.
Most cooks understand this, at least on a subliminal level. After all, what would a salad taste like dressed only with oil? It's the vinegar that makes vinaigrette. And think of the way just a squirt of lemon elevates the flavor of a simple piece of broiled or grilled fish.
How many times have you deglazed a roasting pan with red wine? It's not just the fruit flavor you're after, but the acidity. Cooking down tomatoes in a pot of sauce or soup has much the same effect.
Sweet and tart
If you've heard waiters go on about a "gastrique" served with a dish; it's basically a syrup of boiled vinegar and sugar. Traditionally, it is used for seasoning dishes in which meat is combined with fruit. Used clumsily (as it too often is these days), it's nothing more than a fancy version of sweet and sour sauce.
I wouldn't think of cooking vegetables without at least tasting for acidity -- a squirt of lemon for sautéed broccoli, a hit of red wine vinegar for summertime ratatouille. And almost every time I cook fruit, there's bound to be a jolt of some kind of citrus to balance the sweetness.
But all acids are not created alike; this is cooking, not chemistry. Any well-stocked pantry should have several to choose from, each having its own character and flavor, in addition to that wonderfully useful tartness.
Start with citrus fruit: Lemons are the most common and probably the most useful because they generally are harvested so early that they don't have much flavor besides their tartness. (Meyers, which are almost always harvested fully ripe, are an exception.) Oranges have a softer, sweeter sourness than lemons, and limes are tart but have a bracing herbaceous quality.
Then there is vinegar, or more appropriately, vinegars. Every pantry ought to have several of them, even if they rarely do. It's funny how even cooks who brag about their assortment of $30-a-bottle extra-virgin olive oils will make do with only a couple of cheap vinegars.
This is silly when good vinegars offer much more variety and typically can be found for less than $15 a bottle (rarities such as true aceto balsamico and aged Vinaigre de Banyuls aside).
Though there are a lot of fancy flavored vinegars out there, you should first concentrate on getting very good examples of the basics before going crazy exploring things like fig-balsamic.
The mainstay acid for every cook should be a very good quality red wine vinegar, one that tastes like an extremely tart but otherwise well-made red wine. Unfortunately, these are hard to find commercially -- most of them taste acrid and chemical.
Fortunately, they are incredibly easy to make at home. Here's the outline: Buy a couple bottles of decent, fruity wine, such as a Zinfandel or a Syrah. Put them in a jar with a bottle of unpasteurized commercial red wine vinegar (or a starter you've borrowed from a friend). Cover the jar with a cloth napkin secured with a rubber band to let in the air and keep out the fruit flies. Leave in a cool, dark corner of the kitchen for a month or a month and a half. That's it.
Good white-wine vinegars are harder to make at home because they oxidize quickly. But the standard restaurant supply model made by Vilux is clean and perfectly acceptable. Or use Champagne vinegar, which is sharp with a subtle sweetness. Rice vinegar is also sweet, but with a rounder texture.
Balsamic vinegar is known for its sweetness, but in a burnt-sugar-caramel kind of way that doesn't fit most culinary purposes (and certainly not salads). It is useful in marinades, though, or brushed on a piece of meat before grilling. Sherry vinegar has a distinctive nutty, wine-like quality and great depth of flavor.
Finally, don't overlook that old American standby, apple cider vinegar. Good ones have terrific fruit character. Look for bottles that are unfiltered or unpasteurized.