The glycemic index a great tool to use because you can pick out foods that you know your body will absorb over a long period of time, which will not only leave you feeling satiated for longer but also make your body expend more energy to break down your meals.
Glucose is 100 on the glycemic index because your mind senses it and your liver absorbs it nearly instantaneously. This can cause a spike in insulin levels, which is why it is dangerous for diabetics to eat sugary foods! It also elevates cholesterol, triglycerides and leptin, which negatively affect your metabolism and can lead to obesity, diabetes and heart disease. 
Glycemic load takes into account the portion of whatever you are eating. It follows logic; you can not only substitute toward foods lower on the index but also choose to eat smaller portions of foods that are higher on the index.
Some sports drinks and candy bars that are made of ingredients high on the glycemic index claim to provide your body with a boost while training. Recent studies have shown that this is false! The quantity of food that you eat can affect your performance, but something lower on the glycemic index is probably still better for you because it will not boost your performance.  One study even concludes that ingesting foods high on the glycemic index hampered fat breakdown.
Foods are scored on the Glycemic Index based on how long it takes people to break them down.
Researchers at the University of Sydney maintain a database of scores and provide lots more information about this dieting tool: http://www.glycemicindex.com/about.php