Chocolate is basically made of cacao, cocoa butter, vanilla, sugar, and occasionally milk. The percentage of each of these elements in the chocolate is one of the basic differentiators of one chocolate from another. On chocolate packages these days, producers note the percentage of cacao on the label. The rest of the ingredients in the chocolate make up the remaining percentage. For each recipe, we indicate the percentage of cacao we think would be best.
Chocolates also have particular flavors, depending on the bean and region of origin and how it is processed. The flavors can range from fruity to earthy to coffee or nutty. Chocolates made of a single variety of beans from a particular region emphasize the difference in these flavors. Other chocolates are blends of beans chosen by the chocolatier to achieve a particular flavor profile. The most cautious choice would be to try a sample before you buy enough for the recipe to make sure you like its particular flavor profile.