PG = Propylene Glycol
VG = Vegetable Glycerin
These are the two base ingredients that make up an e-liquid, along side flavourings and nicotine. It's what makes your vape, vape.
VG provides a smoother experience and provides much larger clouds of vapour. It's also extremely viscous compared to PG which has a sweeter undertone but provides a slightly harsher experience on the throat, commonly referred to as throat hit, which helps replicate the sensation of smoking.
Some tanks require PG based liquids, whereas others may require VG based liquids. You can usually tell which of the two liquids a tank requires by the size of the coil.
Smaller coils, with smaller absorption holes on the side, require the thinner PG liquids. Using VG liquids in these tanks can cause you to experience dry hits/burning taste as well as knacker your coil much faster.
Tanks that use larger coils with larger absorption holes on the side require the thicker VG liquids. Using PG liquids in these tanks can cause leakages as well as being very harsh on the back of your throat.
Refer to the product description to find out what liquids each tank should be used with.