propylene glycol - USP Grade Propylene Glycol
PG is a 'throat hitter', and makes things get mixed up easier.
vegetable glycerin - USP Grade Vegetable Glycerin
VG is a slightly sweet provider of 'visual effects' (the exhaled cloud or vapor, that is commonly mistaken as smoke by smokers and non-smokers). It also creates the body of the aerosol/steam, vapor if you must, in your mouth.
All You Need To Know About Nicotine @Wikipedia
Nicotine is nicotine and a 'throat hitter'.
I use flavors made by FlavourArt and The Flavor Apprentice.
I don't always use flavors, but when I do, I get them here.