e Juice is the liquid primarily made up of Vegetable Glycerin, Propylene Glycol, Flavorings and other elements depending on your flavor or type of e-juice.
This e-juice fuels the E-Cigarette or sometimes known as Mods which let you control the voltage which controls how much vapor is created by controlling the temperature.
The fluid is quickly boiled and converted to steam that e-cigarette users (also known as vapors) inhale and exhale clouds of water vapor.
The e juice contains the flavorings of your choice to your Electronic Cigarette and may also contain the level of nicotine you choose.
This will creates the water vapor in which you exhale that very much mimics the smoke from regular cigarette.