Castor sugar can be prepared at home by grinding granulated sugar for a couple of minutes in a food processor or coffee grinder. Please be aware that if you make your own caster sugar in your food processor it will more than likely scratch up the plastic bowl.
Caster or castor sugar is the name of small grains of white sugar in Britain, in terms of size it is between granulated and icing sugar (powdered, confectioners sugar).
It is called caster sugar as the grains are small enough to fit through a sugar "caster" or sprinkler. It is often referred to as superfine sugar or baking sugar in the US.
Because of its fineness, when used in baking, caster sugar creams more easily into butter which gives more volume to the mixture, resulting in extra-light cakes. Caster sugar is also perfect for light and crisp melt-in-the-mouth meringues as it dissolves more quickly than regular white sugar.