Icing sugar is a fine, powdered form of white sugar. Its texture is ideal for making icing and buttercreams as it dissolves quickly and easily and does not really require the application of heat.
Icing sugar mixture is also known as confectioners' sugar. This icing sugar is great to use when making icing for kids' birthday cakes or dusting sweet treats. Similar in appearance to the former, pure icing sugar is also made from pulverised granulated sugar but contains no corn flour.
For everyday cooking, icing sugar mixture does the trick; the only place where pure icing sugar is a must is when making royal icing, modelling fondant and the like. If you are gluten intolerant, be sure to choose pure icing sugar over icing sugar mixture.