Fractional distillation involves boiling crude oil so that the various types of hydrocarbon molecules present vaporize, each in turn. The vapors are then condensed into separate batches according to type.
Crude oil is made up of a large variety of hydrocarbon molecules, each of a different size, weight and boiling point. Shorter, simpler molecules such as methane and propane are gaseous at room temperature. The hydrocarbons used to make gasoline are larger and heavier, and they boil at between 104 and 392 degrees Fahrenheit. Those used to make heating oil boil at between 482 and 572 degrees, while those used to make oil for lubrication boil at between 572 and 698 degrees.
Fractional distillation is only one way of separating and refining crude oil into its various components. Products that are especially in demand, such as gasoline, are often supplemented by cracking larger hydrocarbons in smaller units or unifying smaller molecules into larger ones.