Fractional distillation is a means of splitting crude oil (which is a mixture of a huge range of hydrocarbons in varying amounts) into its fractions (mixtures of hydrocarbons with very similar chain lengths). This is important because different fractions have different properties and therefore have different uses - bitumen has a very large number of carbons per molecule, and is used for tarring roads, whereas petrol has a much smaller number (usually around 8 carbons per molecule).
The process works on the basis that different molecule lengths have different boiling points. This is because for a liquid to boil, the intermolecular forces have to be overcome with the input of energy - longer molecules have more points for forces to act on, so more energy is required to separate them. Henceforth, generally, as the hydrocarbon gets longer, its boiling point increases.
In fractional distillation, the crude oil is piped into the bottom of the fractioning tower, where it is vapourised at a tremendous heat. The gas rises up the tower and as the temperature decreases further up (it is set up to have a temperature gradient), the fractions condense at different points and are piped into separate tanks - the longer ones condense closer to the bottom because they have higher boiling points, whereas the shorter ones remain gaseous for a longer time. Some isn't even vapourised at the start, and is piped-off from the bottom to form residue.
I hope this helps.
