Also referred to as e liquid, juice or smoke juice, it is a liquid oil that is made to be consumed by way of vaporization. E Juice is a mixture of Vegetable Glycerin (VG) and Propylene Glycol (PG). Typically the juice will contain about 80-90% VC and the rest will be PC. Included in the recipe can also be some food grade flavoring and or nicotine. Most e liquids, especially those made in the USA are more or less safe to use but there is still a lot of reseach being done to figure this all out.
The rise of e liquids really began as an alternative to smoking. It's purpose was a new, technological way to consume nicotine but because the nicotine levels within the liquid can be controlled, the juice has also become a good way for some people to ween themselves off nicotine. Imagine starting with the usual level of nicotine in the juice, and then every week or two you replace the e juice with one of lower nicotine levels until you finally get to zero. And then at that point you just need to get rid of the physical habit.
From its orgins, e juice and vaping became something beyond the usual need for a ciggarette. Vaping became something of a quasi sport, not really, but you will find all sorts of vapor competitions that revolve around the biggest smoke clouds or coolest smoke tricks. With e liquid and a powerful box mod, vapers can create and blow huge puffs of smoke, and some really talented individuals are know for doing cool tricks with these vapor clouds. Blowing O rings was once a thing, but those are now amateur hour and you will find hours of video on YouTube around smoke tricks far more comlicated than a single O ring.
There really is no other way to consume e liquids other than a vaporizer or box mod. Either built in or available as a separate attachment, the oils are placed into some sort of heating atomizer or chamber. The chamber is powered by a battery in order to heat the chamber and liquid inside which will create the vapor. Below are a list of some quality vaporizers and mods that can be used with oils and e liquids.
E Juice across the board is very similar. The main differences are the ratios of VG and PG, the flavor, where it is made, and the brand name. Most people like to start with the origin of the product and it goes without saying that USA made juice is the most popular and trustworthy. Some people have a brand they like and clearly is the go to if they have one, but for those searching around, once you narrow it to US made, the next step is choosing the VG content and the flavor. Many users like to go for 100% VG because they think it is safer than a product that contains some PG, but the difference is minimal. So flavor is very important and that will help determine where and what is bought. Of course you'll need nicotine in the juice if that is what you are looking for, but typically the biggest variable is a users taste preferences.
The list of flavors available is never ending as people continue to develop new flavors by combining the flavors already in existence. From brand to brand the same named flavor can easily vary. For instance some options may have a deeper and more profound watermelon flavor than others. Or some flavors come with crazy names but in the end taste similar to something more basic like apple or lemon.