Quality baking chocolate should be naturally gluten-free and made from only a handful of ingredients - unsweetened cocoa (also listed as cacao or cocoa solids), cocoa butter (the fat portion of chocolate), and sugar. It sometimes also includes milk solids and/or milk fat, flavoring (such as vanilla), and an emulsifier (such as soy lecithin). That's it.
Chocolate gets classified along a spectrum of increasing bitterness and intensity of flavor. The major factor in determining its classification is the balance of cocoa and sugar.
On one end of the spectrum you have milk chocolate, which is typically not used in baking. It contains higher levels of milk fats and solids, and lower percentages of cocoa. This makes it better for eating.
On the other end of the spectrum is unsweetened chocolate (sold in bar form and as unsweetened cocoa powder). It contains no sugar, and is nothing but 100% cocoa. It unsuitable for eating, and is used only in baking. We use only natural unsweetened cocoa powder, which is naturally acidic and an important reactant with leavening agents such as baking soda. Cocoa powder is also sold in a second form – Dutch-processed or alkalized cocoa powder. In this form, the cocoa powder has been treated with an alkali to neutralize the chocolate’s natural acidity. If used in leavened chocolate cakes, the baking soda must have another reactant, or you must use baking powder.
In between you'll find semi-sweet and bittersweet chocolate, which we use frequently in our baking. Bittersweet chocolate has less sugar than semi-sweet, and typically contains 60-85% cacao, resulting in a rich, intense (and more bitter) chocolate flavor. Semi-sweet chocolate has more sugar than bittersweet, and typically contains 40-62% cacao. (In the United States, both types of chocolate must contain at least 35% total cacao.)
Finally, there is also white chocolate, which contains cocoa butter, but no cocoa solids. This gives it a rich, buttery consistency, but without the cocoa solids, it contains no real chocolate flavor.
Coming tomorrow: a look at the gluten-free status of the baking chocolates from Ghirardelli, Nestle, and Baker's. Until then!