E-Liquid (or vape juice) is comprised of three (sometimes four) ingredients. Propylene glycol (or PG for short), vegetable glycerin (VG), flavouring and if you choose it - nicotine. Each ingredient plays an important part in your vaping experience.
PG is an odourless liquid which is there to carry the flavour of your vape juice, it also gives you the “throat hit” feeling you’d expect from conventional cigarettes. As it’s a thinner consistency to VG it’s easier to vaporise as well. It’s used in things like ice cream and food enhancers for the same purpose - to deliver maximum flavour. VG is a thick, sweet liquid which is there to provide you with vapour production, it’s also smoother on the inhale. VG is used in a variety of things from caramel to soap to toothpaste.
The flavouring varies massively and there’s a huge range of taste combinations you can try, from desserts to tobacco, fruit to menthol. As you’d expect, it’s there to give you an enjoyable vaping experience.
The final ingredient is nicotine, the addictive ingredient you find in cigarettes. You can get e-liquid without nicotine if you’re in it purely for the joy of vaping but generally the people who are transitioning from smoking to vaping prefer to have some nicotine in there.
Nicotine is available in two forms - freebase which is much harsher on the throat when you go above 6mg and then nicotine salts which are designed to deliver a smooth experience no matter the strength and can go as high as 20mg/ml (2%). You can also purchase hybrid nicotine, which is a combination of the two and provides a smooth experience with a slightly more pronounced throat hit. The strength of nicotine on the bottle is how much nicotine you get per 1ml of juice.