Cocoa powder is a natural product made from chocolate liquor which has had most of its cocoa butter removed. The removal of this vegetable fat leaves a very thick, dark paste which is then dried and ground to make the powder; Dutch process cocoa powder is further processed through the addition of an alkali to remove more of the powder's acidity. Regular cocoa powder is lighter in colour with an assertive flavour; Dutch process cocoa powder is darker and milder.
Cocoa powder contains from 10 to 24% cocoa butter and no sugar, so by itself it's very bitter. However, its addition to chocolate treats made with baking chocolate will give a luscious extra-rich flavour to many sweet desserts.
Cocoa powder has a limited shelf life, and better quality brands will have an expiry date marked on the package. Like all chocolate products, it should be kept in a cool, dry, dark location.