There are two types of cocoa powder: natural and dutch processed. Natural cocoa powder is made from the solids of a roasted, dried cocoa bean, which are very finely ground into a powder for packaging. The powder is slightly acidic and has a very sharp taste to it, though it usually contains fruity or smokey notes just like dark roasted coffee beans do. Natural cocoa powder will react with leavening agents like baking soda, which depend on the presence of an acidic ingredient to activate them.
Dutch process cocoa powder is natural cocoa powder that has been alkalized to remove its acidity and make it neutral. For many, this deepens the flavor of the cocoa powder because sugar (or other ingredients) don’t have to overcome the slightly acidic taste of the cocoa before enhancing its flavor. It also means that dutch processed cocoa powder will not react with baking soda in the same way that non-alkalized cocoa powder does; there should be no additional leavening effect when using dutch process cocoa in a recipe that uses baking soda.
When it comes to baking, the general rule is that if a recipe doesn’t specify, it calls for natural cocoa powder and it will specifically state it if it was designed with dutch processed cocoa powder in mind.
That being said, the reality is that natural and dutch processed cocoa powder are mostly interchangeable. There are very few recipes that would be thrown off by the presence or absence of the acidity of cocoa powder. In fact, many of the ingredients you regularly use in baking are slightly acidic (have a pH < 7) so even recipes that seem to rely on the acidity of cocoa powder to produce leavening are getting that acidity from milk, butter, egg yolks, honey (sugar is neutral, in case you were wondering), etc., and the recipe should turn out to be just fine if you use dutch process instead of natural cocoa powder, or vice versa.