According to Wikipedia:
An electronic cigarette (e-cig or e-cigarette), electronic vaping device, personal vaporizer, digital vapor device or electronic nicotine delivery system is a battery or USB powered device which simulates tobacco smoking.
Electronic cigarette sales increased from 50,000 in 2008 to 3.5 million people in 2012. Among grade 6 to 12 students in the United States, those who have ever used the product increased from 3.3% in 2011 to 6.8% in 2012. Annual sales of smokeless tobacco products now exceed $2.93 billion globally.
A preliminary analysis of e-cigarette cartridges by the US (FDA) in 2009 identified that some contain tobacco-specific nitrosamines (TSNAs), known cancer-causing agents.
The FDA’s analysis also detected diethylene glycol, a poisonous liquid, in a cartridge and nicotine in a cartridge claimed to be nicotine-free.
According to WHO “consumers should be strongly advised not to use" E-Cigs. A number of organizations including the Centers for Disease Control and Prevention, the International Union Against Tuberculosis and Lung Disease, the American Academy of Pediatrics and the Food and Drug Administration have concerns that e-cigs might increase addiction to and use of nicotine and tobacco products in children.
Most concerning part – Tobacco companies have shifted more advertising to e-cigarettes. E-cigarette advertising is widespread on the Internet the ads were placed on websites with the highest average percentage of a youth audience.