PG is propylene glycol. Safe for human consumption but not yet approved by the FDA.
VG is vegetable glycerin. Also safe for human consumption but not yet tested for vaping.
Keep in mind however that the steak you get from a butcher might not be FDA "approved". This don't mean it's unsafe. Just not FDA approved.
I've heard some people have allergic reactions to PG like headaches, sore throat. VG would be a good alternative for these folks. Also, PG is said to have more flavor and less vapor were as VG has more vapor and less flavor. Most vapers seem to mix the two.