Granulated sugar is a refined white sugar containing pure sucrose. Chemically, it is the same as superfine (also known as caster or castor) sugar, though the crystals are not as fine. Typically one type may be substituted for the other.
Superfine sugar more easily dissolves in cold liquids. This can help to make meringues of a finer texture. In some cases, this property is undesirable. For example, the coating on Saucepan Fudge Crackle Cookies is specified to use regular granulated sugar; superfine sugar might dissolve.