I really should just sent you to Exploratorium which has an excellent section on sugar chemistry, but sugar chemistry is so cool, that I have share it with you myself. In fact sugar chemistry is so interesting that it supports a whole sector of the food industry. I am, of course, talking about candy. Food Scientists typically divide the candy industry into chocolate and non-chocolate candy and most of the non-chocolate candy is made from different forms of sucrose.
If you remember sugar is the common name for sucrose, which has the very confusing chemical name of β-D-fructofuranosyl-α-D-glucopyranoside which is a fancy way of saying that it is made up of glucose and fructose. In this post, I will interchange the words sugar and sucrose, but in chemistry sugar refers to saccharides which are considered to be small carbohydrates.
The most important property of sucrose is its high solubility over a wide temperature range. The different sugar candies are made by heating a sugar-water solution, as the heating time increases water evaporates. This increases the sugar concentration and the temperature of the solution increases. The temperature of the solution is dependent on sugar concentration:
Effect of Sucrose Concentration on Temperature
Candy technologists not only control the sugar concentration by heating the sugar-water mixture to a predetermined temperature, they also control the final physical arrangement of the sugar molecules by the other ingredients added and by the way they treat the sugar-water mixture while it is cooling. This determines whether the solution sets in a crystalline form or not. The non-crystalline form is also known as amorphous. Thus sugar candies are divided into crystalline and amorphous.
Crystalline candies include fudges, fondant and rock candies. Amorphous candies include cotton candy, hard candy and brittles, where the sucrose has been set into a glass, and caramel and taffy, which are chewy rather than hard.
References
For Figure:
Wikipedia Candy#Sugar Stages
Harold McGee On Food and Cooking 2nd Edition p681
Lab Cat 
Do you by chance no how long sugar can be left at room temperature before going bad?
What about sugar dissolved in unheated room-temperature water.
Hi Mitch,
Sugar in a bag or container with no water will not really go bad. There are not enough water or nutrients for micro-organisms to grow.
Sugar-water however will go bad because as well as the water there are dissolved nutrients in the water. It depends on the final sugar concentration whether microbes can grow or not.