Cream of tartar is an acidic, white crystalline powder. It is the common name for potassium hydrogen tartrate (KC4 H5 O6). The acidic potassium salt of tartaric acid is used as the leavening agent in baking powders. An impure form, called tartar or argol, forms naturally during the fermentation of grape juice into wine and crystallizes in wine casks.
Cream of tartar is obtained when tartaric acid is half neutralized with potassium hydroxide, transforming it into a salt. Grapes are the only significant natural source of tartaric acid. Cream of tartar is produced from sediment produced during the process of making wine. (The journal Nature reported some years ago that traces of calcium tartrate found in a pottery jar in the ruins of a village in northern Iran are evidence that wine was being made more than 7,000 years ago.)
If you are beating eggs whites and don't have cream of tartar, you can substitute white vinegar (in the same ratio as cream of tartar, generally 1/8 teaspoon per egg white). It is a little more problematic to find a substitute for cream of tartar in baking projects. White vinegar or lemon juice, in the ratio of 3 times the amount of cream of tartar called for, will provide the right amount of acid for most recipes. But that amount of liquid may cause other problems in the recipe, and bakers have found that cakes made with vinegar or lemon juice have a coarser grain and are more prone to shrinking than those made with cream of tartar.