GI rates how fast carbs break down glucose and increase blood sugar levels. a higher GI means faster processing. higher GI carbs throughout the day create those peaks and valleys you want to avoid. also, different foods have different GI's.

Here's a very comprhanesive list of the GI in foods

Basically you wanna stay away from hi gi carbs unless they are post workout and the carbs to have post workout should illicit a good inulin spike such as glucose, dextrose/molto dextrin

http://wave.prohosting.com/rmendosa/gi.htm

Thanks Babyfaced

anytime mate