PG stands for Propylene Glycol, and VG stands for Vegetable Glycerin. They are both added to e-liquids and are the main 2 ingredients found in all bottles. They are represented by a ratio number which you may have seen before (e.g 50/50, 70/30).
These ratios always add up to 100! Whatever isn't VG will be PG, and the majority of e-liquid concentrates/flavourings are PG based. 

The 1st number in these ratios represent the VG %, so can be used to determine how thick that e-liquid is.

VG provides a thicker vapour density compared to PG, and higher VG based e-liquids are designed for DTL vaping (click HERE to read our blog on MTL/RDTL and DTL styles). 

70/30 or 80/20 are the desired ratios for DTL vapers, whereas MTL vapers tend to stick to 40/60 or 50/50 (with 50/50 being the most common available on the market)

And as a general rule (but not always) a 50ml or 100ml bottle will be higher in VG, and the small 10ml bottles will be lower in VG.