Fractional distillation is a process of separating a mixture of chemical compounds. This means that each part (called a "fraction") of the mixture can be kept apart from the other chemicals. Different chemicals have different boiling points. Fractional distillation is done by heating the mixture so that it evaporates and then each fraction condenses in its own separate compartment. The reason we don't use simple distillation is because for liquids that boil at less than 25°kelvin from each other, some of the other liquid(s) may evaporate and so part of the separated liquid may have part of the other liquid(s).