Glycemic index (GI) is defined for each type of food.
Glycemic load (GL) is calculated based on the amount of carb in the food you eat and the quantity you eat (portion size).
Glycemic load actually depends on glycemic index and is calculated with the following formula:
GL = (gram of carbs in food) x GI / 100
What does it mean for your food choices?
This means, that even if you eat food with low GI you may end up eating a meal with high GL if you eat too much.