From Wikipedia:
Liquid
Liquid for producing vapor in electronic cigarettes, known as e-juice or e-liquid, is a solution of propylene glycol (PG) and/or vegetable glycerin (VG) and/or polyethylene glycol 400 (PEG400) mixed with concentrated flavors, and optionally, a variable percent of a liquid nicotine concentrate.
and:
PEG 400 (polyethylene glycol 400) is a low-molecular-weight grade of polyethylene glycol. It is a clear, colorless, viscous liquid. Due in part to its low toxicity, PEG 400 is widely used in a variety of pharmaceutical formulations. More recently, it has been used to make "E-Liquid" for electronic cigarettes (personal vaporizers).
Chemical properties
PEG 400 is strongly hydrophilic. The partition coefficient of PEG 400 between hexane and water is 0.000015 (log), indicating that when PEG 400 is mixed with water and hexane, there are only 15 parts of PEG 400 in the hexane layer per 1 million parts of PEG 400 in the water layer.[1]
PEG 400 is soluble in water, acetone, alcohols, benzene, glycerin, glycols, and aromatic hydrocarbons, and is slightly soluble in aliphatic hydrocarbons.
I've never knowingly used a juice with PEG400. It seems like all the reputable vendors (that I know of) use (or at least claim to use) PG and/or VG as their base.