Crude oil is a mixture of hydrocarbons. The crude oil is evaporatedand its vapours condense at different temperatures in the fractionating column.
As you go up the fractionating column, the hydrocarbons have lower boiling points, lower viscosity (they flow more easily) and higher flammability (they ignite more easily).
The heavist hydrocarbons are distillated first at the bottom of the column at high temperatures of up to 350oC. These are molecules can be used for bitumen, diesel and even kerosene (plane fuel).