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.
Powdered sugar, also known as confectioner's sugar or icing sugar, is granulated sugar that has been ground into a very fine powder.
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.
Caster or castor sugar has a larger particle size, up to approximately half that of granulated sugar.
Hope that helps!
Icing sugar is sugar that has been made fine more like dust.
Using for icing cakes, very smooth consistancy.
Caster sugar is more ruff.
Using for sprinkle over some cakes as a decoration.
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.
Castor sugar as far as I know is simply finely ground sugar crystals (sucrose).
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.