Cream of tartar helps stabilize whipped egg whites, prevents sugar from crystallizing and acts as a leavening agent for baked goods.
Cream of tartar will also help prevent your dough from turning green or blue! Yes, no joke. You can substitute cream of tartar in a lot of recipes, but sometimes you will really need the genuine product. Cream of tartar is a natural stabilizer and acidity regulator
Commercial Cream of Tartar is scraped off the barrels, heated, and then pulverized and processed. They do not have to use any bleaching or other harsh agents.
Attention: just like with Baking soda and Baking powder, you will find in Europe mixes where cream of tartar is already mixed with the other ingredient to make it 'baker ready'. The moment you see something like 'bakpoeder' or 'backpulver' behind the name, you know you have the 'complete' powder.