Cocoa powder is made by grinding cocoa solids remaining after much of the cocoa butter is pressed out. Natural cocoa is acidic; Dutch-process cocoa is treated with alkali and is usually darker and less acidic.
The choice can affect flavor, color and leavening. Whether a recipe uses baking soda, baking powder or both depends on the acidity of the entire formula, not cocoa alone. Use the specified type unless the recipe supports substitution. Cocoa powder is unsweetened unless the label says otherwise; sweetened drinking-chocolate mixtures are a different product.
Reference: Further reading.