Chocolate Wiki
Cocoa powder

Cocoa Powder, sold as natural cocoa powder, is the end result from mostly separating fat free cocoa solids and fatty cocoa solids (Cocoa Butter).

Cocoa powder gives an intense chocolate taste and is available in “Dutch-processed” (alkalized) or natural varieties. Natural cocoa powder is light brown, with a strong, pronounced chocolate flavor.

It is slightly acidic, so it is best to use natural cocoa powder in recipes calling for baking soda. It is also used to make Hot Chocolate and no-bake cookies.  Alkalized cocoa powder is darker in color, less acidic, and has a milder chocolate taste. Alkalized cocoa powder is recommended for recipes that call for baking powder.

Alkalization is performed on the cocoa beans before roasting typically. The cocoa beans are soaked in an alkalizing agent such as potassium carbonate. In lower quality cocoa powders the cocoa beans are often not fermented properly and then alkalized to reduce acidity and increase the classic "chocolate" like flavor.