Vinegar is the common name for a solution including a noticeable amount of acetic acid, the substance that gives vinegar its distinctive flavor. Most table vinegars contain about 5% acetic acid; as with alcohol, a small amount can made a significant difference in the finished beverage.
The acetic acid that characterizes vinegar is created by a family of acetic acid bacteria, which convert the alcohol that results from primary fermentation of sugar by yeast into acetic acid in a secondary fermentation process. The vinegar-producing bacteria form colonies that can create a film or mass in the fermenting liquid; this mass is known as a vinegar mother or mother of vinegar, and vinegar makers often use pieces of it to inoculate new batches.
Because acetic acid bacteria metabolize alcohol, wine is particularly vulnerable to their action, and vinegar made with wine is one of the most common types available. However, almost any substance containing sugar can be and probably has been made into vinegar, if not intentionally than through unintentional fermentation and spoilage.
Vinegar is simple to make at home, although few home cooks go through enough vinegar to justify maintaining their own mother.