Cream of tartar is the common name for potassium hydrogen tartrate, an acid salt that has a number of uses in cooking. Now, before you get all jittery about the thought of cooking with an acid, it's worth noting that lettuce, brown sugar, steak, plums, and just about every other food we eat is acidic. In fact, egg whites, baking soda, and milk are the only non-acidic (alkaline) foods we have.
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, and cream of tartar is obtained from sediment produced in 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.)
Cream of tartar is best known in our kitchens for helping stabilize and give more volume to beaten egg whites. It is the acidic ingredient in some brands of baking powder. It is also used to produce a creamier texture in sugary desserts such as candy and frosting, because it inhibits the formation of crystals. It is used commercially in some soft drinks, candies, bakery products, gelatin desserts, and photography products. Cream of tartar can also be used to clean brass and copper cookware.
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.
Now, if they were making cream of tarter 7,000 years ago in Iran (or at least if cream of tartar was making itself), don't you think you can find the small plastic or glass bottles it comes in among the hundreds of other small jars and bottles in the spice section of your grocery store? Or you can get it in almost any quantity online.