What is the difference between caster sugar and refined sugar?

Caster sugar granules are smaller than refined sugar granules. It's basically crushed white sugar. And then, icing sugar is even smaller than castor sugar. In the UK refined means white sugar of any shape, form or description, although the Word is almost never used in connection with sugar. Castor sugar is a very small grit that is used mostly for cooking - being finer it is easier to incorporate it into cake mixtures etc. - it can be brown or white although fine brown sugars are more usually called soft.