True balsamic vinegar begins with the juice of Trebbiano grapes, must be aged for a minimum of twelve years in wooden casks, and must be approved by a consortium of master tasters. It's a dark, shiny liquid that's as thick as molasses. Small bottles of this tradizionale balsamic vinegar start at about $75 and go upwards of $400. Tradizionale balsamic is best used sparingly: Drizzle a bit over risotto, sprinkle a few drops on ripe fruit, or sip a thimbleful after dinner. This is not the balsamic called for in most recipes.
Condimento-grade balsamic, also called salsa balsamica, salsa di mosto cotto, is made in the traditional way but doesn't bear the stamp of consortium approval. These vinegars may be released prior to twelve years and so do not qualify. Prices for these vinegars range from $18 to $40, and are the best value for the savvy consumer.
Most supermarket-shelf balsamics are simply red-wine vinegar, sweetened and darkened with cooked grape juice or caramel coloring and flavoring (or both). Despite not being the real thing, some taste good and are fine for marinades and vinaigrettes.
True balsamic bears the name Aceto Balsamico Tradizionale di Modena or di Reggio Emilia on the label. "Tradizionale" is the key word here. In the supermarket, look for balsamics made without added sugar or caramel color. Taste a few brands to determine which you like best.
Keep balsamic vinegar capped or corked and away from light and heat.