Crude oil is a mixture of lots of different oily substances. Luckily for us, they all boil at different temperatures. So we can separate them in a distillation tower (or still).

The thinner, runny ones have lower boiling points. They get all the way to the top of the tower before they condense. It is cooler at the top of the tower.

The thicker, sticky substances have higher boiling points. They don't get far up the still.