Cracking is used to turn long chain hydrocarbons into more useful short chain HC's by breaking the chain. As a consequence of this, unsaturation is introduced, hence Alkene products formed.
Process is driven by a favorable change in entropy, hence the higher temperatures used.
Reforming is for example when you take a chain and give it a circle ish structure (I.e. aromatic ring or something of the sort). Do stuff to it to give it a higher octane number bro.
In context of AS Chemistry, techniques like these are used to improve the usability of the fuel, for example, increasing the octane number, increasing volatility, decreasing activation energy. Thing like that.