Red wine vinegar, as its name implies, is vinegar made from red wine.
Other vinegars may substitute—white wine vinegar and sherry vinegar are good choices—but the acidity level will vary, so adjust amount used to taste.
Vinegar's flavor can have a big impact on the finished dish, so try different brands to see which you like best. Look for a balance of acidity and sweetness, a good depth of flavor that's assertive without being astringent.
Keep in a cool, dark place. Sealed it will keep indefinitely. Once opened, it should last about 6 months.