Dark chocolate and white chocolate both contain cocoa butter and are eaten as dessert or used in confectionery.
Chocolate is derived from the bean of the cocao (cocoa) plant which breaks down in to chocolate liquor (the ground or melted state of the nib of the bean), cocoa butter (the fat component) and cocoa powder (the non-fat part of the cocoa bean ground into a powder). Dark chocolate is produced by adding cocoa butter to sugar and cocoa powder. Unlike milk chocolate, dark chocolate does not contain any milk solids.
White chocolate contains only cocoa butter, sugar and milk solids and no chocolate liquor or cocoa powder. So technically, white chocolate is not really chocolate at all.
edit Proportion of components
The cocoa content of commercial dark chocolate can vary from 90% to 30% (sweet-dark). Common terms used to distinguish different grades or content of dark chocolate bars are bittersweet, semi-sweet, and sweet dark chocolate.
White chocolate typically contains 20% cocoa butter (by weight) at least 14% total milk solids, at least 3.5% milk fat, and less than 55% sugar or other sweetener.
edit Taste and texture
Since dark chocolate does not contain milk solids it has a more pronounced chocolate taste. However, the lack of milk additives means that the chocolate is more prone to a dry, chalky texture with a bitter aftertaste. Flavors like vanilla are often added to dark chocolate.
Both dark and white chocolate are sold in bars either plain or with added flavors such as vanilla, orange, mint and nuts. For professional candy making, chocolate has to be tempered first (heated, chilled then reheated briefly). Both types of chocolate are used in desserts either melted or as chips, to flavor coffee and other beverages, in sauces and dips, although white chocolate is used less so.
edit Health benefits
Eating dark chocolate has well known positive effects on health. Studies show that eating a single 1.4 ounce bar of dark chocolate can lower hormone levels that cause emotional stress and anxiety. Due to the high contents of flavanol antioxidants in the cocao bean, dark chocolate has benefits for cardiovascular health and lowers blood pressure.
White chocolate on the other hand does not have the same health benefits. It is a source of empty calories with a standard 1.4 ounce bar having 225 calories and lots of sugar and saturated fast. However, as it does not contain chocolate it is caffeine free so ideal for those who are sensitive to caffeine.