What is cracking and how is it done?

Cracking of CRUDE OIL is the breakdown of long-chain hydrocarbons (produced from fractional distillation) to shorter-chain hydrocarbons, which are more economically useful. 

Long chain hydrocarbons are vapourised (turn into gas) and passed over an Aluminium Oxide catalyst at a temperature of 500C. 

Products include ethene (for plastics) as well as petrol (for fuel)