Superfine sugar is also known as bar sugar (so-named for use in mixing alcoholic drinks) or baking sugar. It is granulated sugar with unusually small crystals. Sold as caster sugar or castor sugar, this is the standard type of sugar in some parts of the world.
Superfine sugar crystals are easy to disolve in cold liquids. This property also causes doughs, meringues and batters made with superfine sugar to be of a finer texture than is usually achieved with granulated sugar.
Superfine sugar is not the same as powdered sugar, confectioners' sugar, and icing sugar. A blender can be used (abused) to create a decent approximation of superfine sugar from granulated sugar. Depending on the recipe, it may be acceptable to simply substitute either granulated sugar or powdered sugar.