Natural unsweetened cocoa powder is very bitter and gives an intense chocolate flavor to baked goods. It is acidic and when used with baking soda, it creates a leavening reaction causing the batter to rise when baked.
Dutch processed cocoa is mild in flavor and reddish-brown in color. It is treated with an alkali to neutralizes it’s acidity and will not react with baking soda. Instead it must be used in recipes calling for baking powder, unless there are other acidic ingredients (like buttermilk) being used.
For more information, David Lebovitz has comprised a FAQ about natural and dutch process cocoa powders. You can find the answers to questions like “Can I use cocoa powders interchangeably in recipes?” or “What is the best brand of cocoa powder?” over here.