Vaping and E-Cigs 101
Propylene Glycol or PG is a watery, clear substance that is used in everyday household items like toothpaste, shampoo, conditioner, as well as many foods, and even inhalers. In vapor it gives the feeling of a "throat hit". This is the feeling of actually inhaling something in addition to air. Too much propylene glycol can be irritating to the throat, this is described as being "too harsh" for the user. However, in practice, this feeling is a combination of the nicotine, PG, and amount of power your device is delivering.
Flavoring comes from many sources. While there are many thousands of flavors available most manufacturers today have "red flagged" a few specific substances and the reputable manufacturers have removed anything "not safe for inhalation" from their formulas. They prove this with sheets showing tests have been performed by a third-party laboratory certifying them free of or low in specific substances such as Diacetyl and Acetyl Proprionyl. These substances are generally used to provide a "buttery" flavor, but most companies have found alternatives.
Nicotine is of course why we are all here. Some companies use nicotine extracted and filtered from tobacco plants. This is pure nicotine with no additives. Some use a lab-made synthetic nicotine commonly known as TFN or tobacco-free nicotine. Others use nicotine salts which are sourced using a different nicotine extraction process from the nicotine plant. In the end, they are all nicotine. Each E-Juice flavor comes in different levels of nicotine measured in MG/ML or milligrams per milliliter, so you the consumer can decide which level is right for you. The most common levels these days are 0MG, 3MG, 6MG, and 12MG of nicotine. Most heavy nicotine users start at 12MG/ML or 6MG/ML (if using a larger device), but the sales numbers don't lie - most users end up moving down to 3MG/ML eventually. This is the most commonly sold nicotine level these days, and the lowest before 0MG, which is of course no nicotine at all.
The most important things to know about e-liquid are what nicotine level you want to use, and what ratio of PG/VG you want in your e-liquid. The most common are 70% VG/30% PG, or 80% VG/20% PG. It used to be that 50/50 was most common, but as vapers started wanting bigger clouds and a smoother vape the VG numbers rose and the PG levels fell. All e-liquid will work in any tank, RDA, or RTA. However the bigger the vapor production of the device, the lower you are going to want your nicotine level to be. Otherwise it can lead to too harsh of a throat hit, as you are getting 2x or 3x the vapor production, therefore you are getting 2x to 3x the nicotine.
External battery devices use 18650 lithium-ion batteries. These are high-powered batteries made for quick bursts of high power output. These are not AA or AAA batteries that you put into a remote control. The biggest benefit of having external batteries, aside from extending the lifespan of your device, is that when your batteries are dead you can have a second set waiting in your charger, and you can swap them out to have immediate full use of your device. I carry several batteries with me throughout the day in case my device runs out of power.