source: http://en.wikipedia.org/wiki/Couverture_chocolate
Couverture Chocolate is a high quality chocolate that contains extra cocoa butter (32-39%). The higher percentage of cocoa butter, combined with the processing, gives the chocolate more sheen, firmer "snap" when broken, and a creamy mellow flavor.
The total "percentage" cited on many brands of chocolate is made up of some combination of cocoa butter to cocoa solids (cacao). In order to be properly labeled as "couverture", the percentage of cocoa butter must be between 32-39%, and the total of the percentage of the combined cocoa butter plus cocoa solids must be at least 54%. Sugar makes up the inverse percentage, and up to 1% may be made up of vanilla, and sometimes soy lecithin.
Couverture is used by professionals for dipping, coating, molding and garnishing.
Do not confuse the term "couverture chocolate" with "confectionery chocolate", "compound chocolate" or "summer coating": These products have a lower percentage of solids (cacao), and they may also contain vegetable oil, hydrogenated fats ("trans fats"), coconut and/or palm oil, and sometimes artificial chocolate flavoring.
Some brands of couverture chocolate are packaged tempered, and others are packaged untempered. Subsequent tempering may or may not be required, depending on the usage and the desired characteristics of the final product.
Do not substitute couverture when semi-sweet, bittersweet chocolate, or unsweetened chocolate is called for in a recipe. The increased cocoa butter content and the sugar content may alter the finished product.