The Glycemic Index or GI describes this difference by ranking carbohydrates according to their effect on our blood glucose levels. You want to be eating foods that has a Low Glycemic Index (GI). Low GI foods produce only small fluctuations in our blood glucose & insulin levels. Keeping us fuller for longer. It's a slow release process.
However, If you eat a chocolate (High GI) your blood glucose & insulin levels spike quite sharply giving you that "Good" feeling... this feeling is very short lived. This causes you to Crave more of the High GI foods. It's like a "fix" that you need. So eating foods that has a High GI will make you feel full for a very short period of time.
Foods that has a High GI, are the foods you want to try and steer clear from.
eg white bread, chips, chocolates mashed potatoes, etc