Sugar crystals, particularly white sugar, may come in different granulations. Some common types are:
* Icing: very small crystals that quickly dissolve in liquids or can be used for decorating desserts, like confectioners' sugar
* Caster: larger crystals than icing
* Granulated: basic table sugar, with larger crystals than caster or icing
* Preserving: very coarse sugar used as a preserve in jams and similar confections
http://recipes.howstuffworks.com/sugar1.htm

There are several speciality white sugars:

* caster sugar is just a very small crystal size white sugar
* icing sugar is ground up white sugar, essentially sugar dust
* sugar cubes are lumps of sugar crystals "glued" together with a sugar syrup
* preserving sugar is a special large crystal
http://www.sucrose.com/ltypes.html

“Regular” or white sugar, extra fine or fine sugar
“Regular” or white sugar, as it is known to consumers, is the sugar found in every home’s sugar bowl, and most commonly used in home food preparation. White sugar is the sugar called for in most cookbook recipes. The food industry stipulates “regular” sugar to be “extra fine” or “fine” because small crystals are ideal for bulk handling and not susceptible to caking.

Fruit Sugar
Fruit sugar is slightly finer than “regular” sugar and is used in dry mixes such as gelatin and pudding desserts, and powdered drinks. Fruit sugar has a more uniform small crystal size than “regular” sugar. The uniformity of crystal size prevents separation or settling of larger crystals to the bottom of the box, an important quality in dry mixes.

Bakers Special Sugar
The crystal size of Bakers Special is even finer than that of fruit sugar. As its name suggests, it was developed specially for the baking industry. Bakers Special is used for sugaring doughnuts and cookies, as well as in some commercial cake recipes to create a fine crumb texture.

Superfine, ultrafine, or bar sugar
This sugar’s crystal size is the finest of all the types of granulated white sugar. It is ideal for delicately textured cakes and meringues, as well as for sweetening fruits and iced-drinks since it dissolves easily. In England, a sugar very similar to superfine sugar is known as caster or castor, named after the type of shaker in which it is often packaged.

Confectioners or powdered sugar
This sugar is granulated sugar ground to a smooth powder and then sifted. It contains about 3% cornstarch to prevent caking. Powdered sugar is ground into three different degrees of fineness. The confectioners sugar available in supermarkets – 10X – is the finest of the three and is used in icings, confections and whipping cream. The other two types of powdered sugar are used by industrial bakers.

Coarse sugar
As its name implies, the crystal size of coarse sugar is larger than that of “regular” sugar. Coarse sugar is recovered when molasses-rich, sugar syrups high in sucrose are allowed to crystallize. The large crystal size of coarse sugar makes it highly resistant to color change or inversion (natural breakdown to fructose and glucose) at cooking and baking temperatures. These characteristics are important in making fondants, confections and liquors.

Sanding sugar
Another large crystal sugar, sanding sugar, is used mainly in the baking and confectionery industries as a sprinkle on top of baked goods. The large crystals reflect light and give the product a sparkling appearance.

http://www.sugar.org/consumers/sweet_by_nature.asp?id=275