What is Electronic cigarette ?

An electronic cigarette exhibiting another common design, that of an ordinary cigarette.An electronic cigarette or "e-cigarette" is a battery-powered device that provides inhaled doses of nicotine by delivering a vaporized liquid nicotine solution. It is an alternative to smoked tobacco products, such as cigarettes, cigars, or pipes. In addition to nicotine delivery, this vapor also provides a flavor and physical sensation similar to that of inhaled tobacco smoke, while no tobacco, smoke, or combustion is actually involved in its operation.

An electronic cigarette usually 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. A common design is also 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.
 
A disassembled cigarette-shaped electronic cigarette.
A. LED light cover
B. battery (also houses circuitry)
C. atomizer (heating element)
D. cartridge (mouthpiece).When a user inhales through the device, air flow is detected by a sensor, which activates a heating element that vaporizes a nicotine solution stored in the mouthpiece.It is this vapor that is inhaled by the user. On some models an LED on the opposite end of the device is also activated during inhalation, which serves as an indicator of use, and simulates the glow of actual burning tobacco.

While electronic cigarettes take many forms, they each generally employ the same basic components: a mouthpiece, a heating element, a rechargeable battery, and various electronic circuits.

Mouthpiece ("cartridge")
The mouthpiece is a small disposable plastic cup affixed to the end of the tube. Inside, the piece contains an absorbent material that is moistened with a flavored nicotine liquid solution.[2] The mouthpiece is referred to in the industry as a "cartridge". When the liquid in the cartridge has been depleted, it can either be refilled by the user, or replaced with another pre-filled cartridge.

Heating element ("atomizer")
The heating element serves to vaporize the liquid in the mouthpiece so that it can be inhaled. This component is referred to in the industry as an "atomizer".
 
An electronic cigarette battery connected to a USB charger.
Battery and electronics
Most electronic cigarettes employ a lithium-ion rechargeable battery to power the heating element. Battery life varies depending on the battery type and size, frequency of use, and operating environment. Many different battery charger types are available, such as wall outlet, car, and USB. The battery is generally the largest component of an electronic cigarette.

Most electronic cigarettes employ an electronic airflow sensor to automatically activate upon inhalation, while some models instead have a manual button that the user must press while inhaling. Various other electronic circuits are usually employed as well, such as a timed cutoff switch to prevent overheating, and an LED to signal activation of the device. An orange-colored light is often chosen for its visual resemblance to actual burning tobacco in the front of the cigarette.

While some larger electronic cigarette models employ an independently replaceable standard-size battery cell, many models are too small to house a standard-size cell and instead require a proprietary component made by the electronic cigarette manufacturer. For those models, the battery and electronic components are housed within a single replaceable part, which is generally referred to in the industry simply as the "battery" — even though it actually contains many other components as well.

Nicotine solution
Nicotine solutions usually consist of nicotine diluted in either propylene glycol or glycerin, both common food additives. They also commonly have a variety of different flavorings added.

Solution is available in a variety of different flavors and nicotine concentrations. Nicotine concentrations range from high doses (to mimic the content of regular cigarettes) to midrange and low doses (that mimic the nicotine content of "light" and "ultralight" cigarettes). Solutions are also available which contain little or no nicotine.

Some flavor varieties attempt to resemble traditional cigarette types, such as regular tobacco and menthol, and some even attempt to mimic specific cigarette brands, such as Marlboro or Camel. Solutions are also available with other flavorings, such as strawberry, orange, mint, vanilla, caramel, coffee.