Cocoa powder has more fat than carob powder, and some caffeine. Chocolate chips often have less cocoa butter than chocolate bars, which helps them retain their shape better when they're baked in the oven. The curls are fragile, so it's best to move them around with a toothpick. If the chocolate crumbles, it's too cold. Nutella is a popular brand.
Cocoa is similar to unsweetened chocolate, only it's in powdered form and has less cocoa butter.
Cooks like it because it allows them to make low-fat goodies, or to use fats other than cocoa butter. Cocoa's also used to dust candies and cakes. European process cocoa is treated with an alkali, making it milder yet richer-tasting. It's the preferred cocoa for beverages and frozen desserts, and for dusting baked goods. You can often substitute one type of cocoa for the other, but if the recipe includes baking soda, it may be counting on the acid in natural cocoa in order to react.