Volatile Acidity (VA) is simply an organic acid that evaporates from the wine. There are a number of different acids that can create this, but around 96% of VA is Acetic Acid.
Too much VA can cause the wine to taste vinegary. But in the right doses it can often give a lift to the wine, making the aromas feel sharper, fruitier and cleaner. Many Barolos have high VA and the renowned 1947 Cheval Blanc is widely recognized to contain high levels of volatile acidity. So like many problems a small amount is good but a large amount is not. The sensory threshold for acetic acid is >700 mg/L, with concentrations greater than 1.2-1.3 g/L becoming unpleasant.
The most common form of VA is Acetic Acid which is caused by bacteria known as Acetobacteria. In the presence of oxygen the Acetobacteria reacts with the alcohol to form Acetic Acid and is the reason why wine that has been opened for a while can taste of vinegar. VA can also come from the alcoholic fermentation when the wine is being made. Wines where the yeast struggles can cause the yeast to create Volatile Acids. Sweet wines in particular can have a harsh gingery edge that can catch in the back of the throat, which is a sign that the yeast has had a problem dealing with high sugar levels.