Chocolate is found in many different forms, ranging from solid to pre-melted, from extra dark to white, from unsweetened to sweet and milky. All chocolates, even those of same type, taste different, depending on the quality and roasting of the cocoa beans, the quality and style of production.
* Unsweetened Chocolate
After cocoa beans have been processed and roasted, they are ground to form this purest form of chocolate. It contains no sugar, has a bitter, full chocolate flavour and is used mainly in manufacturing chocolate products. In its liquid state, unsweetened chocolate is also known as chocolate liquor (though it contains no alcohol).
* Bitter, Bittersweet, Semisweet, and Sweet Chocolate
These types of chocolate are made by combining unsweetened chocolate with sugar and flavorings such as vanilla or vanillin. These chocolates vary widely, from manufacturer to manufacturer, depending on the amount of sugar, additional cocoa butter, lecithin, and flavorings they contain. They are very dark brown in colour.
* Milk Chocolate
Milk chocolate is made with dried milk powder. It has a much milder, more creamy flavour than dark chocolate and cannot be substituted for bittersweet or dark chocolate in baking and dessert recipes because it has a lower cocoa solid content. Extra care should be taken when melting it.
* White Chocolate
Technically, white chocolate is not chocolate at all because it contains no chocolate liquor. It is made from cocoa butter, milk, and sugar. As with milk chocolate, it is sensitive to heat, so be very careful when melting it.
* Cocoa Powder
Cocoa powder is made from chocolate liquor that has had nearly all of the cocoa butter removed by hydraulic pressure. The pressure forms what is called a press cake, which is then ground into powder. Dutch-processed cocoa is alkalized, which means that an alkali is added to the chocolate during processing to produce a darker, reddish colour but a slightly milder flavour. Non-alkalized cocoa powder is usually lighter in color than alkalized, but it delivers a stronger chocolate flavor.
(Source: 1001 Reasons To Love Chocolate – Barbara Albleft & Mary Tiegreen)