Castor is a fine sugar, sold in the US as "superfine." It is finer than granulated sugar but coarser than powdered sugar and dissolves well in liquids, making it ideal for use in merengues. You can make your own by putting granulated sugar in a food processor for a minute or two, though let the sugar settle for a few minutes before opening the food processor- this method creates sugar dust.
Grainy, sticky(when wet), fine
Selecting and Buying
Conserving and Storing
Keep in a dry, cool place in a sealed container. In hot countries, sugar can attract ants into your cupboards. If the sugar becomes hard it can be broken up by putting the bag in the microwave on high for about 1 minute.