A fraction refers to a portion of a thing and distillation means separation. Through the process of fractional distillation, crude oil is separated into its fractions. In its raw form, crude oil is a viscous, sticky and dark liquid, which is not of much use. To extract real value from crude oil, we have to separate the constituents, which are later used in different ways.
In fractional distillation, the crude oil is heated boiling temperatures. When it boils, the constituents get gasified. These gases are then passed through a fractionating column, which is a tall tower, with many condensers at different heights. As we go higher in the fractionating column, the temperature goes down.
The level at which a fraction condenses depends on its boiling point
Fractions which have high boiling points, cool down and condense at lower heights, while those with low boiling points condense towards the top.
So, we see that fuel oil, diesel oil, kerosene have higher boiling points and they are the first ones to condense in that order. Naptha and gasoline have lower boiling points and they condense at higher levels in the fractionating column.
The smaller a hydrocarbon molecule, the higher it rises before it condenses. Hydrocarbons which have lower molecules like petrol are better fuels than say, diesel, because they are more volatile, can be ignited easily and flow easily.