Vinegar is dilute acetic acid. Take any drink that is fermented to produce alcohol - grapes, apples, etc. and you get wine or cider or whatever. If the wrong bugs get in, they will keep the fermentation going until all the alcohol has been converted to acetic acid. THAT is called vinegar. Wine vinegars are useful acidulants in cooking, Apple Cider Vinegar is one of the most useful health aids you can find.