During fractional distillation, what are we seperating and how are we seperating them?

During fractional distillation, the process of heating a mixture is used to seperate fractions. Each fraction has a different boiling point. As we begin to heat the mixture, the substances that have a lower boiling point begin to boil. These are then cooled into a seperate flasks (the distilate). As we increase the temperature, we distill off different fractions depending on their boiling points.