Only two raw materials go into making plain (dark) chocolate: cocoa beans (including additional cocoa butter from the beans) and sugar. In general, as cacao content goes up, sugar goes down, and usually the quality of the chocolate goes up as well, since that is what you should be tasting. The optimum range of cacao content is 55-75%. Anything lower will be too sweet to taste the chocolate; anything higher will be too bitter, regardless of the quality of the cacao. The key to good chocolate is not only good beans, but using the right amount of sugar for a particular bean. And since sugar is much cheaper than cacao, the proportions also will affect price.
The two main varieties of beans are the fairly rare and less hardy criollo, with its more sought-after flavor profile, and the more vigorous, full-bodied forastero. The forastero is often roasted longer and hotter, resulting in a burnt flavor, which helps mask its flavor flaws. When blended together, as is common, the two beans complement each other. Most top quality chocolates use only a small amount of forastero – to minimize the burnt flavor and maximize body and depth of flavor. Criollo gives acidity and complexity, like a good pinot noir.
Vanilla is commonly used in chocolate, although the majority of producers use artificial vanilla (ethyl vanillin, extracted from a variety of conifer tree), which lacks the complexity of flavor in real vanilla.
Most eating chocolate (vs. unsweetened baking chocolate) also contains an emulsifier or stabilizer, usually soy lecithin. It is added during the conching stage to prevent separation of the cocoa butter and cocoa solids, and to give smoother texture and appearance (versus a thick, grainy texture).