PG - Propylene glycol. Pretty thin and watery. Promotes and transports flavor. Increases throat hit a bit. Found in many daily things, food flavorings, tooth paste, concert fog machines, breathing inhalers, the list is long
VG - Vegatable Glycerin. Thick and syrupy. Creates more vapor than PG. Sweetens and dulls throat hit a bit. Extracted from various vegetable sources, palm oil, nuts, seeds, etc.
Depending on your atomizing device will determine the extend of what ratio of these 2 base transports will be. Clearomizer like an EVOD, slow wicking you'd need a 50PG/50VG to 70PG/30VG. Newer Tanks on the market now can handle 30PG/70VG to 50/50. Rebuildable dripping atomizers you could get away with 100%VG.