there are afaik 2 standard bases. And several more non standard ones.
VG (vegetable glycerine) has calories, mutes flavor ever so slightly, produces a lot of “smoke”, is the primary ingredient in the fluid used in theatrical smoke machines, tends to be somewhat more resistant to developing an allergy (though it can happen) and has sort of the consistency of honey.
PG (propylene glycol) is thinner, produces less vapor, is more likely to cause an allergic reaction, is the only base used in disposables, has a clearer flavor, and has the consistency of light oil.
PeG (polyethylene glycol) is the nontoxic antifreeze, is primarily for people allergic to PG
Ethanol often used at quite high molarity, will get you drunk quite quickly, vapes hot (it’s boiling temperature is below water but still really quite high) is the tiniest base. Even thinner than water. Produce NO visible vapor and will reduce the visible vapor of other bases, making it sometimes used in stealth vapes
Water too hot, boils at a high enough temp to hurt, but as can be added in small amounts to other bases. Produce NO to almost no visible vapor and can reduce the visible vapor of other bases. So much so that a few drops of water added to a juice is an easy way to make stealth juice