Cracking breaks carbon-carbon bonds to turn heavier hydrocarbons into lighter ones. This can occur thermally (as occurs during the petroleum formation process beneath the earth) or through the action of a catalyst:

Thermal Cracking
Steam, visbreaking, or coking
Catalytic cracking
Fluid catalytic cracking (FCC) cracks heavy oils into diesel and gasoline. Uses a hot fluid catalyst.
Hydrocracking (similar to FCC but lower temperature and using hydrogen as catalyst) cracks heavy oils into gasoline and kerosene
A catalytic reformer converts naphtha into a higher octane form, which has a higher content of aromatics, olefins, and cyclic hydrocarbons. Hydrogen is a byproduct, and may be recycled and used in the naphtha hydrotreater.
Steam reforming is a method of producing hydrogen from hydrocarbons, which may then be used in other processes.
Solvent dewaxing removes heavy wax constituents from the vacuum distillation products.