Red wine vinegar is vinegar made from any type of red wine through fermentation of alcohol molecules into acetic acid. Red wine vinegar is consumed in a variety of ways, most commonly in salad dressings or marinades for meat and vegetables.
Red wine will ferment into vinegar on its own if left exposed to oxygen for at least a month. This process can be sped up through the addition of acetic acid-producing bacteria to red wine. These bacteria convert the natural sugars in alcohol to acetic acid. Because the high calorie sugar is converted to acid, vinegar is essentially a calorie-free food.