Dark chocolate is the chocolate that is darker than the regular one (
milk chocolate).... and white chocolate.
Consists of two types, semisweet (used for cooking) and bittersweet (more cocoa, vanilla). However, both can be referred as 'couverture', that is chocolate rich in
cocoa butter
It has more antioxidants than milk or white chocolate. However, avoid anything with caramel,
nougat or other fillings. These fillings are just adding sugar and fat which erase many of the benefits you get from eating the chocolate.