I was going to write a great post and try to give a brilliant description of what an electronic cigarette is, but I found this excellent description at Wikipedia.  Why re-invent the wheel?  So hear it is:

An electronic cigarette, e-cigarette or personal vaporizer, is a battery-powered device that provides inhaled doses of nicotine or non-nicotine vaporized solution. It is an alternative to smoked tobacco products, such as cigarettes, cigars, or pipes. In addition to purported nicotine delivery,[1] this vapor also provides a flavor and physical sensation similar to that of inhaled tobacco smoke, while no smoke or combustion is actually involved in its operation.

An electronic cigarette takes the form of some manner of elongated tube, though many are designed to resemble the outward appearance of real smoking products, like cigarettes, cigars, and pipes. Another common design is the “pen-style”, so named for its visual resemblance to a ballpoint pen. Most electronic cigarettes are reusable devices with replaceable and refillable parts. A number of disposable electronic cigarettes have also been developed.

That’s pretty much it in a nut shell.  I couldn’t have said it any better.  Thanks Wikipedia!