Cocoa powder is made from roasted and ground cacao seeds from which much of the fat has been removed. It is naturally tart and acidic. A little cocoa adds plenty of chocolate flavor.
There are two types of cocoa: natural (nonalkalinized) and Dutch Process (alkalinized). Natural cocoa powder (also called unsweetened) is simply untreated cocoa powder; it is rarely labeled with the word natural on the package, but will simply say cocoa. Dutch Process cocoa ― so named because a Dutch man invented the process –– has been treated with an alkali to reduce its harshness and acidity. Along the way, "dutching" gives the cocoa a rich dark appetizing color, mellow toasted flavor, and coffee notes.
If a recipe simply calls for cocoa, use natural cocoa. Because Dutch Process cocoa is more alkaline, it may alter the chemistry in a recipe, reacting differently than natural cocoa with baking soda or baking powder. In recipes with no leaveners, natural and Dutch-process cocoa are interchangeable.
It's best to store cocoa away from herbs and spices and other aromatic substances, as it picks up other flavors relatively easily.