An electronic cigarette (e-cigarette) is a device that allows users to inhale nicotine by heating a vapour from a solution that contain nicotine, propylene and flavorings.
There is no burning involved, there is no smoke like a traditional cigarette.
But while they have been branded as carrying a lower risk than cigarettes, increasing levels of studies are showing health dangers.
E-cigarettes do not produce tar or carbon monoxide, but the vapor does not contain any harmful chemicals.
Nicotine is the highly addictive chemical which makes it difficult for smokers to quit.
Nearly three million people in Britain use e-cigarettes, and more than nine million Americans.
1. Standard e-cigarette
Battery-powered device containing nicotine e-liquid.
It vaporizes flavored nicotine liquid.
Very similar to normal e-cigarettes but with sleeker design and a higher concentration of nicotine.
Thanks to its 'nicotine salts', manufacturers claim one pod delivers the amount of nicotine as a pack of cigarettes.
It is composed of an e-cigarette (battery and temperature control), and a pod of e-liquid which is inserted at the end.
The liquid contains nicotine, chemicals and flavorings.
Like other vaping devices, it vaporizes the e-liquid.
3. IQOS by Philip Morris
Pen-shaped, charged like on iPod.
It is known as a 'heat not burn' smokeless device, heating tobacco but not burning it (at 350C compared to 600C as normal cigarettes do).
The company claims this method lowers users' exposure to carcinogens from burning tobacco.