The glycemic index measures how fast a food is likely to raise your blood sugar. This can be helpful. For example, if your blood sugar is low and continuing to drop during exercise, you would prefer to eat a carb that will raise your blood sugar quickly. On the other hand, if you would like to keep your blood sugar from dropping during a few hours of mild activity, you may prefer to eat a carb that has a lower glycemic index and longer action time. If your blood sugar tends to spike after breakfast, you may want to select a cereal that has a lower glycemic index.
The numbers below give that food's glycemic index based on glucose, which is one of the fastest carbohydrates available. Glucose is given an arbitrary value of 100 and other carbs are given a number relative to glucose. Faster carbs (higher numbers) are great for raising low blood sugars and for covering brief periods of intense exercise. Slower carbs (lower numbers) are helpful for preventing overnight drops in the blood sugar and for long periods of exercise.
Note that these numbers are compiled from a wide range of research labs, and often from more than one study. These numbers will be close but may not be identical to other glycemic index lists. The impact a food will have on the blood sugar depends on many other factors such as ripeness, cooking time, fiber and fat content, time of day, blood insulin levels, and recent activity. Use the Glycemic Index as just one of the many tools you have available to improve your control.
For additional information on the glycemic index Carb Counting, and blood sugar control, we also recommend Using Insulin, Pumping Insulin or the Pocket Pancreas. You may also want to read about the Satiety Index, How carbs vary, or check out Diet Books.
*Actually, the GI indirectly measures a food's effect on blood sugar. It actually measured the "area under the blood sugar curve" following a set intake of that carb.
To learn carb counting, these books really help: Pumping Insulin, Using Insulin and the Carb and Fat Gram Guide. A Gram Scale for your kitchen makes it easy to accurately measure carbs in cereal, fruit, pancakes, rice, pasta, and more.