The Not So Technical Version
Propylene Glycol (PG) is one of the main ingredients used to create your E-Liquids, it is primarily used to thin down the Vegetable Glycerin (PG). It has a faint sweet taste and is colourless and odourless. PG can sometimes cause a dry throat when used too often, if this is the case with yourself, then what others do is to use distilled water to thin down the VG.
Vegetable Glycerin is too think for an atomiser, therefore it needs to be thinned down, be sure to read the post on Vegetable Glycerin.
The Technical Version
Propylene glycol is a clear, colorless and hygroscopic liquid. Propylene glycol contains an asymmetrical carbon atom, so it exists in two stereoisomers. The commercial product is a racemic mixture. Pure optical isomers can be obtained by hydration of optically pure propylene oxide.
Propylene glycol is a component in newer automotive antifreezes and de-icers used at airports. Like ethylene glycol, the freezing point of water is depressed when mixed with propylene glycol owing to the effects of dissolution of a solute in a solvent (freezing-point depression) glycols are good for this purpose as they are cheap, non-corrosive and have very low volatility.
The acute oral toxicity of propylene glycol is very low, and large quantities are required to cause perceptible health damage in humans; propylene glycol is metabolized in the human body into pyruvic acid (a normal part of the glucose-metabolism process, readily converted to energy), acetic acid (handled by ethanol-metabolism), lactic acid (a normal acid generally abundant during digestion), and propionaldehyde (a potentially hazardous substance).