Superfine Sugar is a finer grade than granulated sugar which means that it dissolves much more quickly than granulated. You can use it in place of granulated white sugar in the same amount with no problems. You won't have sugar granules sitting undissolved at the bottom of your mixing cup or glass.
Don't use powdered or confectioner's sugar. This stuff has cornstarch added to prevent clumping which makes your drink taste like, well, cornstarch.
Superfine sugar can be found at the grocery store. It goes by a variety of other names such as baker’s sugar, bar sugar, berry sugar, and, in Britain, castor sugar. On this site I usually refer to it as superfine sugar.
Keep your sugar in a tightly sealed container, as it is prone to clumping (especially in a humid environment). I keep mine in a glass canning jar.