An electronic cigarette looks like a cigarette made from tobacco but actually has just one thing in common with the real thing: nicotine.
An electronic cigarette typically consists of a white cylindrical tube with a brown filter and a red, glowing tip. Instead of tobacco, however, e-cigarettes are filled with liquid nicotine. When smokers draw on the business end of the tube, a battery heats the nicotine, which creates a water-based nicotine vapor that is inhaled into the lungs. The excess cigarette-like "smoke" vapor is then emitted from the end of the e-cigarette, completing the cigarette smoking illusion.
See also: Electronic Cigarettes as Quit Aids?