Balsamic vinegar comes from an Italian vinegar making process dating back to the middle ages. There are two main types.
Traditional balsamic vinegar is made only with one ingredient “grape must” (in Italian, “mosto”), the sweet juice of freshly pressed grapes that is boiled to a concentrate, fermented and acidified, and aged for 12 to 25 years or longer in wood barrels. A highly crafted product, traditional balsamic vinegar is produced in small batches. It is sweet, tart, dark, syrupy.
Modern commercial balsamic vinegars (what you will likely find at your local grocery store) combine concentrated grape must with wine vinegar to speed up the acidification process. This vinegar is typically aged from 2 months to 3 years in large oak barrels. Mixing grape must with wine vinegar allows producers to make a high volume of balsamic vinegar much more efficiently than using the traditional method. Depending on the mix of sweet grape must and tart wine vinegar, balsamic vinegar can vary in its sweetness. It can range in consistency from thin to syrupy.