The liquid used in electronic cigarettes, often called E-Liquid, is usually a base mixture of Propylene Glycol (PG) and/or Vegetable Glycerin (VG) with flavoring and/or nicotine added according to preference. As the vapor is inhaled by the user they are able to both taste the flavoring and absorb the nicotine that is diluted in the E-liquid, if any.
Both PG and VG are considered GRAS (Generally Recognized As Safe) by the FDA and EPA. They are common ingredients found in many of the foods we eat, cosmetics we use, medications we take, and common household items such as toothpaste, mouthwash, and soap. They are also used in the fog machines used in theaters and night clubs.
PG is a very thin liquid that dilutes and carries flavors very well, making it the optimal base for mixing, but when thinned out even further by the addition of flavoring and nicotine it may produce less vapor. Also, a very small percentage of the population experiences a sensitivity to PG that may present itself in a number of ways from throat irritation to skin irritation. However, most people with this sensitivity are still able to use liquids that contain a lower amount of PG without any negative effects.
VG is a slightly sweet and very thick liquid in its pure form, which results in much higher vapor production than PG. However, the downside to this thickness is that it may cause wicking, clogging, or burning issues in some devices if the VG concentration in the liquid is too high. Also, due to the inherent sweetness of VG it tends to alter or dull the flavor slightly and reduce the amount of throat hit experienced by the user.
Ultimately, the E-Liquid mixture percentages of each user are merely personal preference based on the tradeoff between flavor, throat hit, and vapor production. However, it is recommended for most devices that no more than 15-20% VG be used in the E-Liquid to provide a boost in vapor production without the negative effects of higher VG concentrations.