Caster or Castor sugar is finely granulated white sugar. It is most commonly found in North America and Britain. This kind of sugar is used for icing, essentially for its quick dissolving property.
It is made by dissolving and purifying raw sugar content in various chemicals. While undergoing many refining processes, it is made superfine easy enough to dissolve in any liquid.
Castor sugar is either made from sugar cane or sugar beet. The sugar granules are so small that they fit through castor, a form of sieve. Hence it is called castor sugar. It is also popularly used in baking and mixing drinks.
However, it is not so superfine sugar like the icing sugar or confectioners’ sugar. The latter is more thoroughly powdered and has little starch content that keeps it away from clumping.