Glycemic Index (GI) is a rating system for carbohydrates that deals with how quickly the sugar enters the blood stream and the extent of the insulin response following that entry. GIs were initially established to help diabetics regulate insulin levels following meals. Those carb sources that have low GIs generally enter the blood stream slower or cause a smaller insulin response. This can be beneficial for those trying to lose fat as well as those who are diabetic.
see the Misc.Fitness.Weights FAQ: what is the "glycemic index?
Rick Mendosa maintains an extensive list of the glycemic indices of foods at http://www.mendosa.com/gilists.htm .