It's typically nicotine, VG/PG (essentially glycerines like you'd use in a fog machine, but medical/food grade), and often a small amount of food flavoring concentrate suspended in the same VG or PG base.

I quit smoking with this stuff and eventually dropped off the nicotine. I made my own liquid because I could source the ingredients myself, control how much nicotine I used, and knew exactly what I was puffing on...something I couldn't say when I still smoked.