Cracking and reforming

Cracking is to break the long hydrocarbon molecules into smaller molecules. This can be done by thermal process under high pressure, or catalytic (under high temperatures and in the presence of a compound that facilitates the chemical reaction). When hydrogen is involved, it is called hydrocracking, is when water is called steam cracking. 

The reforming to convert naphtha to produce gasoline or premium. 

There are other processes refining as isomerization, alkylation, etc. ... We can thus influence the characteristics of the products obtained (octane, color, odor, volatility ...).