Confectioner's sugar and icing sugar are identical to powdered sugar, while caster sugar and superfine sugar have a slightly coarser grind. The number of X's on the package also indicate how finely the sugar has been ground, from coarser XX to the finest 10x. Most regular powdered sugars that we buy at the grocery store are ground to 10x.
The biggest gain we get from using powdered sugar is that it dissolves easily without needing to be heated. This is how we can make a simple icing from just powdered sugar and milk. It's also why we use it to dust baked goods — it literally melts on our tongues.
Plus, of course, powdered sugar looks pretty dusted over any dessert. Here are a few recipes that put this sugar to particularly good use:
• Mexican Spiced Wedding Cookies • Fluffy Vanilla Marshmallows • Coconut-Gooseberry Clafouti • Peppermint Cream-Filled Butter Cookies • How to Make Crème Brêlée at Home