Castor sugar is powdered sugar. It's very fine. Like used on powdered doughnuts. I've gotten similar results sending plain white sugar through a coffee grinder.

         Its the stuff that Fidel Castor used to sell to the Russians.

         Castor sugar is what we call "superfine" sugar. This actually came from Britain and it's called castor sugar because the grains of the sugar are so small, so fine, that they will fit through a "castor" or a sprinkler. Because it is so fine, it tends to dissolve more quicly than regular sugar. Because of this, you will probably see a lot of recipes for cold drinks, like iced tea, call for this type of sugar. On a side note, if you don't have any castor / superfine sugar on hand, you can make your own by putting regular sugar into your food processor for a couple of minutes--works like a charm!!