Each Booze Joulie takes the form of a stainless steel ice cube-shaped shell, inside of which is sealed "a proprietary blend of food-grade liquids." This blend has a lower freezing point than that of water. This means that it takes longer to freeze – about three hours in a typical freezer – but more importantly, it also takes longer to thaw.
When it does thaw, of course, it's still contained within the cube. In the event that it does eventually find its way out (something which is said to be very unlikely to happen), it is reportedly non-toxic and safe to consume.
Booze Joulies are intended for use in any kind of cold beverage, and are available now at a price of US$24.95 for a set of six. They're made by the same people who brought us Coffee Joulies, which are designed to lengthen the amount of time that hot beverages stay warm.
Source: Booze Joulies