There are three basic ingredients for
juice: PG, VG and PEG. (Propyleneglycol, Vegetable Glycerine, Polyethyleneglycol)
PG is the most common one, well tested for human inhalation as well. Some people develop an intolerance for it, though.
VG is much thicker, and therefore in many juices because it makes the juice thicker. Some people develop an intolerance for VG. It's not so well tested for human inhalation, but a very very common additive in food and medicine.
Some people, BTW, are fine
vaping either PG
or VG, but can't tolerate a mix of the two.
PEG is more complicated, because there is not just one sort of it around. It has not been so well tested, either, and some people are slightly suspicious of it. (Do a forum search, I am sure this has been discussed here as well.) It is of course a good thing for those who tolerate neither PG nor VG, but otherwise, I
personally would not get it. Still, it can hardly be worse than smoking analogs.