There are a wide range of uses for confectioners' sugar, making it a useful thing to keep around the house. You may also find this sugar labeled as “powdered sugar” or “icing sugar.” All three styles of sugar are the same; the difference is regional, rather than culinary. You may also want to be aware that most confectioners' sugar has additives like cornstarch which are designed to keep it from clumping; sometimes these additives are unexpected, and you may want to check if you have allergies or you are cooking for people with dietary restrictions.
Confectioners' sugar is a type of sugar which has been ground into a very fine powder. It can be made from either cane or beet sugar; beet sugar is a common choice for making it, since the sugar is so refined that the subtle differences between cane and beet sugar are not as noticeable.
The primary advantage of confectioners' sugar is the fine grain, which allows it to dissolve very quickly. Some recipes call specifically for powdered sugar, since they benefit from a quickly dissolved sugar. Because icing sugar often has additives, you should be wary of replacing regular sugar with this sugar in recipes; the additives may cause the food to perform strangely, and some baked goods actually benefit from the granular texture of regular sugar.
One of the most well known and common uses for confectioners' sugar is in icing. The sugar can be beaten with butter and flavoring to make a quick, creamy butter frosting. It can also be blended with egg whites to make royal icing, or beaten with more exotic ingredients like cream cheese for unique frostings. Some people also like to dust confectioners' sugar over baked goods like spice cakes for a hint of sweetness without a dense frosting.
Confectioners' sugar is also useful for things like meringues, which require sweetness but would collapse under the weight of regular granulated sugar. This sugar will merge seamlessly with the ingredients in the meringue, and the small amount of added cornstarch actually helps to support the meringue so that it will not sag or get weepy. This sugar is also used in things like rum balls and truffles.
There are also non-dessert uses for confectioners' sugar. For example, sugar and cinnamon are often sprinkled together on Middle Eastern dishes like b'stilla and other foods with phyllo dough to bring out the flavors of the dish.