See refining info from the American Fuel and Petroleum Manufacturers.

Crude Oil: mixture of many different hydrocarbon substances

Refining: process of separating crude oil into fractions using distillation.

Distillation: Process using boiling point differences to separate different liquids from each other. See Wikipedia distillation article for more info.

Fractions: mixtures of liquids with similar boiling points taken from a distillation column. In the diagram above, “gasoline” and “fuel oil” are some of the fractions.

Light fractions boil easier and are taken from the top of a tower; heavy fractions are more difficult to boil and flow to the bottom of the tower; fractions with middle boiling points are taken from the middle of the tower.