White Chocolate has no cocoa solids and usually is not listed with a %. Good white chocolate should have a high cocoa butter content and no other sources of fat. White chocolate has more sugar than milk, semi-sweet, or bittersweet chocolate. The other components of white chocolate are milk or milk solids and vanilla.
Milk Chocolate has a cocoa content of 36%-46%. Sugar and milk solids make up the rest.
Semi-Sweet Chocolate has a cocoa content of 52%-62%. Semi-sweet chocolate is a good entry level for those who are new to darker, more bittersweet chocolate.
Dark Chocolate has no milk solids added whatsoever and have a cocoa content of 70%-100%.