Baking soda is a pure sodium bicarbonate, when combined with moisture and an acidic ingredient, such as honey, yoghurt or buttermilk, the resulting chemical reaction produces bubbles of carbon dioxide that expand in oven temperatures causing the baked goods to rise.
Cream of tartar is pure potassium bitartrate. It is used as the acidic ingredient with baking powder to trigger the chemical reaction to make the cakes rise.
Baking powder contains both the sodium bicarbonate (baking soda) and the acidifying agent (cream of tartar). It also contains a drying agent, usually a starch. This may contain gluten so it is important that you purchase a baking powder that states it is gluten free.
Xanthan gum is used in gluten free baking to bind, thicken and emulsify gluten free ingredients. It is a corn based product made by fermenting corn sugar with a bacteria called xanthomonas campestris. Omitting xanthan gum from gluten free baking can result in a dry, crumbly cake but overuse can make cakes heavy and gummy.