Ok, the good news first: cream of tartar is completely unrelated to teeth. Whew!
In fact, it's an acidic by-product from the process of making wine. It's found in the sediment left behind in barrels after the wine has been fermented, and it gets purified into the powdery white substance that we use in baking.
One of the best known uses for cream of tartar is for stabilizing egg whites while whipping them. A pinch of cream of tartar added while the whites are being whipped will strengthen the matrix of bubbles and help prevent the egg foam from collapsing too quickly. The tartar also helps to increase the volume of the egg foam and keeps them bright and white.
Cream of tartar is also often added to baked products to help activate the alkaline baking soda. In fact, cream of tartar mixed with baking soda is what gives us baking powder.
One more way cream of tartar gets used in the kitchen is when we're working with sugar. Where cream of tartar is a stabilizing agent for egg whites, a pinch added to boiling sugar is actually an interfering agent. The cream of tartar gets in the way of sugar's natural tendency to bind together and prevents those dreaded sugar crystals from forming.
Some sources say that white vinegar or lemon juice can be substituted for cream of tartar, but it can get tricky since you need more vinegar to get the same acidic reaction and that amount of added liquid could cause problems with the original recipe. We think it's best to stay with the real thing and keep it stocked in the pantry!