PG and VG or Propylene glycol and Vegetable glycerin are two key ingredients in vape juice. They carry the flavour and nicotine and create the vapour you inhale.
Propylene glycol (PG) is a thin flavourless liquid which carries flavour and also delivers a stronger throat hit when vaped.
Vegetable glycerine (VG) is a thicker, sweet tasting liquid that also carries flavour, although less well than Propylene glycol. It creates more vapour than Propylene glycol, so a liquid that is high in VG will create larger vapour clouds.
The majority of e-liquids will use both PG and VG and vary the amount of each depending on the desired vaping experience and the type of device the liquid is designed for. This combination of PG and VG is known as the PG/VG ratio.