This is a white/off-white fine powder, also known as potassium bitartrate. Cream of Tartar is commonly used in baking powder along with sodium bicarbonate to give lift to cakes and is also added to beaten egg whites to improve volume and stability.
Use cream of tartar to stabilize egg whites for meringues, add loft to baked goods, as an alternative to buttermilk in recipes, and prevent sugar crystal formation in homemade candy.