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.