Glycemic index (GI) is a scale from 1 to 100 that ranks carbohydrate-rich foods by how quickly they raise blood glucose levels compared to a standard food.

The "standard food" is glucose (100) or white bread. All foods that contain carbohydrates, such as starchy vegetables (potatoes, corn, peas), desserts, fruits, bread, pasta, and rice, can be tested for how they raise blood sugar levels after being eaten.

Additional Resources:

http://www.diabetes.org/food-and-fitness/food/planning-meals/glycemic-index-and-diabetes.html#What_is_the_glycemic_index