That is a question that many people seem to be confused about. From well-meaning vapers who say “It's just water vapor” (it isn't) to tobacco control fanatics yelling, “We don't know what's in it” (we do), misinformation and weird beliefs about e-liquid are all over the place.
Luckily, there's no real mystery about how e-juice is made or what goes into it. The ingredients are generally all listed on the label. This is the law in the EU and will soon be in the USA, but reputable juice makers have now been doing it for decades anyway.
So it's easy to find out what's in your 100ml ejuice, but is that very informative? Nearly all of its chemical names, which can look intimidating – and they also don't say a lot about what the substances are. That makes it easy for people to produce scare stories, such for instance “E-liquid contains antifreeze” (it doesn't!)
Nearly all of what isn't VG in your liquid might be propylene glycol or PG. Some liquids contain more PG than VG; these are better in clearomisers and older atomizers because they're less viscous.
PG can be an alcohol, like VG, and it is also nearly odorless. It has a light sweet taste, although inadequate for it to be utilized as a sweetener; it will get added to foods, medicines and other products to help keep them moist, though.
An added use for PG is in antifreeze, and it's due to this that many people prefer to claim e-juice contains antifreeze. This is dishonest. PG is just one ingredient in antifreeze, and it's there because it's non-toxic; PG-based antifreeze is less harmful to children and pets. It's worth pointing out that water is also an ingredient in antifreeze; just because something is utilized in a product that has a reputation if you are poisonous, that doesn't say anything about how harmful that substance is.
There were indeed some minor medical issues connected to PG. This has been studied since the 1940s and is classed as generally safe; swallowing or inhaling it won't would you any harm. A small percentage of people are sensitive to it, though, so if liquid with a high PG content makes you feel unwell, switch to one with more VG.
One of the finest reasons for having vaping is the wide range of delicious-tasting juices, and that's all down to flavorings. The liquid doesn't have much taste by itself, so it's flavored with food-grade additives.
If there are any health concerns about vaping that's mostly down to flavorings. There are a few ingredients that are safe to consume but not to inhale – diacetyl, for example. However, the did well at eliminating additives there are worries about. Don't use food flavorings from the supermarket though; many of them contain oils, which are harmful if inhaled. Stick to flavors from vape manufacturers, since these have now been elected for maximum inhalation safety.