There are two base substances for almost all e-liquids on the market: propylene glycol and vegetable glycerine.
The first among these, and my personal preference, is propylene glycol. Propylene glycol, or PG, is a biproduct of petroleum. Many common ice cream brands, cake mixes, frostings, salad dressings, shampoos, toothpastes and even dog food contain the PG substance. It is odorless and clear.
PG has a lower viscosity than vegetable glycerin, or VG, which means the instances of clogging in your vaping device and the instances of having to replace coils are generally reduced. PG-based e-liquids generally produce less of a plume of vapor for those interested in that.
Vegetable glycerin is also contained in many household products including many of those previously listed. VG is more of a gelatinous product which can take a heavier toll on vaping devices; however, generally speaking VG e-liquids are capable of producing larger vapor clouds. They generally offer a gentler throat hit, meaning that users may experience less of a feeling of smoking a conventional cigarette.
It is also important to consider which vaporizer you are using and that is be able to handle the type of e-liquid. For instance, a low wattage vaporizer may not be a good option if you are using it with a VG product. It would take considerably more time and battery consumption to heat efficiently. And the device sustains wear and tear on its components through clogging.
Many more advanced vapers experiment with different variations of each product. If you have a device that can handle it, give it a try and find the right fit for you.