PEG or polyethylene glycol is a synthetic organic compound usually derived from petroleum that is found in many products from medicine to anti-freeze.  It comes modified in many different forms and is labeled as such with its molecular weight or size of the total molecule. PEG 400 is a common mixing agent that was shown when heated to temperatures to produce similar amounts of formaldehyde as found in a cigarette.  

PG or propylene glycol is another synthetic organic compound that can also be used as anti-freeze.  It is one of the most common mixing agents along with VG.  It was also shown to produce formaldehyde and has been known to cause allergic reactions.

VG or vegetable glycerin is very similar to PG in chemical structure but, as the name suggests, is derived from plant oils and created by chemical modification.  This thick liquid is what produces the huge clouds in many products. In the previously mentioned research however VG and MCT did not produce significant by-products like formaldehyde.