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.
Because of its fineness, it dissolves more quickly than regular white sugar, and so is especially useful in meringues and cold liquids. It is not as fine as confectioner’s sugar, which has been crushed mechanically (and generally mixed with a little starch to keep it from clumping).
If you don’t have any castor sugar on hand, you can make your own by grinding granulated sugar for a couple of minutes in a food processor (this also produces sugar dust, so let it settle for a few moments before opening the food processor). You can also purchase castor sugar online.