The battery-powered devices made of plastic or metal heat a liquid nicotine solution, creating vapor that users inhale. Some models are disposable, and some are designed to be refilled with cartridges containing what enthusiasts call "smoke juice." Some e-cigarettes are made to look like a real cigarette with a tiny light on the tip that glows like the real thing. WHAT'S IN THEM: The ingredients in the liquid used in most e-cigarettes include nicotine, water, glycerol, propylene glycol and flavorings. Propylene glycol is a viscous fluid sometimes used as an antifreeze but also as a food ingredient. SELLING POINTS: Users say e-cigarettes address both the addictive and behavioral aspects of smoking. Smokers get their nicotine without the thousands of chemicals found in regular cigarettes. And they get to hold something shaped like a cigarette, while puffing and exhaling something that looks like smoke without the ASH, odor and tar. THE WORRIES: So far, there's not much scientific evidence showing e-cigarettes help smokers quit or smoke less, or to say whether they are safe. Some are concerned that e-cigarette marketing could tempt young people to adopt the habit. GROWING MARKET: The industry has rocketed from thousands of users in 2006 to several million worldwide, leading to the rise of more than 200 brands. Analysts estimate retail and Internet sales of e-cigarettes could reach $2 billion by the end of the year. ARRAY OF FLAVORS: While some e-cigarette makers are limiting offerings to tobacco and menthol flavors, others are selling candy-like flavors like cherry and strawberry -- barred for use in regular cigarettes because of the worry that the flavors are used to appeal to children.WHAT'S AHEAD: The U.S. Food and Drug Administration has said it plans to set marketing and product standards for electronic cigarettes in the near future as part of its oversight of the tobacco industry. The move could limit the marketing levers companies are currently using and restrict sales to minors. It could also limit flavors.