E-liquids (also known as e-juice or vape juice) typically contain 3-4 main elements -- vegetable glycerin (VG), propylene glycol (PG), flavouring and nicotine (optional).
Also read: What's in your vape juice?
VG and PG are solvents. Flavouring provides the tastiness that many vapers (flavour chasers) look for. While there are zero nicotine options, vapers who use vaping as a way to quit smoking prefer juices with nicotine.
With vaping, you're able to adjust the nicotine levels in your vape juices. Heavy smokers might start out with 18mg nic levels and slowly reduce it until it gets to 3mg or even 0.
Also read: What does MG in vape juices mean?