Ganache is a glaze, icing, sauce, or filling for pastries, made from chocolate and cream. It is made by heating equal parts by the weight of cream and chopped chocolate, warming the cream first, then pouring it over the chocolate. The mixture is then typically left to rest for a short period (between 3 and 10 minutes generally, dependent on volume) before it is stirred or blended until smooth, with liqueurs or extracts added if desired.

Source: Wikipedia