Wikipedia
Couverture chocolate is a very high-quality chocolate that contains a higher percentage of cocoa butter (32–39%) than baking or eating chocolate. This additional cocoa butter, combined with proper tempering, gives the chocolate more sheen, a firmer "snap" when broken, and a creamy mellow flavor.
The total "percentage" cited on many brands of chocolate is based on some combination of cocoa butter in relation to cocoa solids (cacao). In order to be properly labeled as "couverture", the product must contain not less than 35% total dry cocoa solids, including not less than 31% cocoa butter and not less than 2.5% of dry non-fat cocoa solids; Couverture is used by professionals for dipping, coating, molding and garnishing.
The term "couverture chocolate" should not be confused with compound chocolate. Products that contain compound chocolate have a lower percentage of solids and contain non-cocoa fats.
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.
Couverture chocolate should not be substituted when semi-sweet, bittersweet, or unsweetened chocolate is called for in a recipe, as the increased cocoa butter content and the sugar content may alter the finished product.