A key part of refineries, distillation towers separate the oil from the wells based on the length of the carbon chains. Extermely high temperatures are able to separate crude into more commonly known gasses such as LPG, naphtha, kerosene, as well as heavy and light oil. Short chains are referred to light crude while long chains are called heavy crude.
Distillation towers are built in layers with the crude entering the tower at the bottom. By heating the crude, it is changed into gases. As the gasses rise, they cool, and then condenses back into a liquid. This liquid is then removed from the distillation tower to be processed further. The longer the carbon chain, the higher the temperature at which the compounds will boil.
Distillation towers do a good job at separating the original elements of crude oil. However this is often not complete enough to pass on to the end user and needs to be refined further. Cracking, reforming, and polymerization are all additional methods for producing downstream products. Cracking further breaks down the longer chains. Reforming changes the molecular structures of low quality molecules. Polymerization takes smaller chains and joins them together to form longer chains.
Image from: The Oil Drum