Glycemic Index 

“ The glycemic index (GI) is a ranking of carbohydrates on a scale from 0 to 100 according to the extent to which they raise blood sugar levels after eating. Foods with a high GI are those which are rapidly digested and absorbed and result in marked fluctuations in blood sugar levels. Low-GI foods, by virtue of their slow digestion and absorption, produce gradual rises in blood sugar and insulin levels, and have proven benefits for health.” 

    http://www.glycemicindex.com/about.php

Although glycemic index (GI) has a very precise mathematical definition, it is something that can be reasonably deduced intuitively. The GI of sugar is 100. Foods that are intact, or more structured, have lower GI. The Wikipedia site about GI is very good and has a nice graph giving examples of GI: Click on the graph to learn more. 

BONUS: When you add protein or fat to a carb, such as in this salad, it decreases, or improves its effective glycemic index. It is a good idea to makes sure there a little protein with every carb.