Cream of Tartar: Is an acidic ingredient that is generally used in combination with baking soda. Cream of Tartar is a byproduct of wine-making and can also be used as a leaven. It can be used as a stabilizer in egg whites and whipped cream to increasing heat tolerance and volume. It can also be used in candies and/or sugar syrups to prevent crystallization.
As discussed previously, cream of tartar in commonly used as a component in baking powder to activate the baking soda. Read more about different types of Leavening Agents here