PG (Propylene Glycol) and VG (Vegetable Glycerin) are odorless liquids that produce a vapor when heated.
Alongside the flavoring and nicotine, e-liquid is made up of a base and just a bit of distilled water to determine how much that base is diluted. This base suspends the flavor and nicotine so that the ingredients don't separate, meaning the base helps you get a perfectly balanced hit each and every time so you don't end up with an overly flavored or overly nicotine heavy vape. However, not every e-liquid uses the same base.
PG and VG e-liquids are popular for different reasons. PG has a stronger throat hit and VG creates billowing clouds. Mixing the two is very common as doing so gives the consumer the best of both liquids.