Since acetobacter requires oxygen to convert ethyl alcohol to vinegar, it tends to form a culture on the surface of the wine, cider, or whatever. Eventually, this culture will form a slimy, cloudy skin at the surface. It may cut off the interaction between the alcoholic liquid, and the oxygen at the surface, if left undisturbed long enough. Thus, when producing a large batch of vinegar, you'll want to periodically stir it. (This will also help oxygen reach acetobacter suspended beneath the surface.) This mass, composed of live and dead acetobacter and its wastes, is called "mother of vinegar". It can be transferred to uncultured batches to start the process anew. Mother of vinegar can be obtained from home brewing enthusiasts, or various winemaking supply merchants.