Castor or caster sugar is the name of a very fine sugar in Britain, so named because the grains are small enough to fit though a sugar "caster" or sprinkler. It is sold as "superfine" sugar in the United States. <br />
Powdered sugar, also known as confectioner's sugar or icing sugar, is granulated sugar that has been ground into a very fine powder.<br />
In industrial food production, it is used where a quick dissolving sugar is required. Domestically, it is principally used to make icing or frosting and other cake decorations. It is often lightly dusted on baked goods to add a light sweetness and subtle decoration.<br />
Caster or castor sugar has a larger particle size, up to approximately half that of granulated sugar.<br />
Hope that helps!
Icing sugar is sugar that has been made fine more like dust. <br />
Using for icing cakes, very smooth consistancy.<br />
Caster sugar is more ruff.<br />
Using for sprinkle over some cakes as a decoration.<br />
Even some foods.
I'm not sure what the exact difference is; but I would think that icing sugar would have a little gelling substance added, so that it retains its shape while still wet, when shaped or extruded. Maybe starch, maybe agar.<br />
Castor sugar as far as I know is simply finely ground sugar crystals (sucrose).<br />
Have a look on a packet - they have to declare ingredients nowadays.
I need to know if there is icing sugar without cornstarch available anywhere, without having to contact an industry source.