What is the difference between dark chocolate and plain chocolate?

Couverture is used mainly by professionals because it is a lot more temperamental to work with. It contains only cocoa solids, cocoa butter, and sugar (and milk solids in the case of Milk Chocolate couverture). In order to be properly labeled as "couverture", the percent of actual cocoa butter must be between 32-39%

I'm not sure what is meant by "plain chocolate." If you just mean chocolate chip kind of chocolate, then that more depends on the brand, but basically it's chocolate with less cocoa butter than couverture (and thus less of the "melt in your mouth" quality).

Although Couverture means 'coating' in FrenchCoating chocolate is NOT couverture. It is used for coating things like candies and uses vegetable oil instead of cocoa butter, so it's very easy to work with, but doesn't have the nice richness of couverture

Couverture chocolate is indeed used mainly by professionals, cooks, and chocolatiers. It is often used for making high quality candies such as candy bars, truffles, and etc. because it melts smoothly, but needs tempering.

Plain chocolate is also known as Dark Chocolate which is a chocolate with out milk. This chocolate is known for it's great health benefits.

Click on the links below for more information about chocolate varieties: