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Stuart - not quite correct....
Jet is essentially the same as kerosine however kero is heavier than gasoline (and lighter than diesel). Jet is often the most valuable product produced in a refinery so I wouldn't call it a waste product from pertrol manufacture. You are correct that jet is used in jet (turbine) engines and is often labled avtur at the airport. Jet is distilled from raw crude and does not contain any cracked product (unless it has come from a hydrocracker in which case the percentage cracked product in the finished jet is limited.
Avgas is a leaded high octane petrol type fuel and typically blended mainly from alkylate and reformate. Both of these components are formed through reaction of other molecules (some of which are distilled from crude). Alkylate is essentially a C8 compound that is produced by combining butane with butylene in the Alkylation process. Reformate is produced by reforming heavy virgin naphtha (HVN which is distilled from crude) HVN is a whole mix of molecules that boil in the range 70-140 degC (so C6 to C10 type molecules). HVN is typically about 70 octane so it is reformed - this changes the shape of the molecule to increase the octane number - typically to 98.
Lead is added to raise the octane number to over 100.
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