The glycemic index is a well-established way of classifying foods based on their potential to raise blood glucose (blood sugar). High glycemic index diets are typically composed of foods that produce a large rise in blood sugar after a meal and cause large amounts of insulin to get released. This causes stress in the body (INFLAMMATION) and causes the body to make new fat cells to store the extra sugar released from the high GI food you just ate.
Eating a big meal of only high GI foods will cause you to feel hungry only a few hours after that meal, because the blood sugar will dive sharply and this low blood sugar causes hunger again. Thus, eating only high GI foods can cause over-eating and weight gain and pre-diabetes. In fact, poor eating habits consisting of a diet of high GI foods is the #1 cause of Type II Diabetes!
For more info on the glycemic index and glycemic load, I like this website’s info. She points out that the glycemic index is only a tool to use, and should not be the only factor in choosing what foods you eat, but it is a good place to start–I agree completely.
For a list of Low, Medium and High GI Foods, check out the table below, taken from here.