E-cigarettes, also known as vapes, are devices that simulate tobacco smoking. They consist of an atomizer, a power source such as a battery, and a container such as a cartridge or tank filled with liquid. Instead of smoke, the user inhales vapor. The atomizer is a heating element that vaporizes a liquid solution called e-liquid, which quickly cools into an aerosol of tiny droplets, vapor, and air. E-cigarettes are activated by taking a puff or pressing a button. Some e-cigarettes look like traditional cigarettes, and most kinds are reusable. The vapor mainly comprises propylene glycol and/or glycerin, usually with nicotine and flavoring.

E-cigarettes produce an aerosol by heating a liquid that usually contains nicotine, flavorings, and other chemicals that help to make the aerosol. Users inhale this aerosol into their lungs, and bystanders can also breathe in this aerosol when the user exhales into the air. E-cigarettes are known by many different names, including e-cigs, e-hookahs, mods, vape pens, vapes, tank systems, and electronic nicotine delivery systems (ENDS) . Some e-cigarettes are made to look like regular cigarettes, cigars, or pipes, while others resemble pens, USB sticks, and other everyday items. Using an e-cigarette is sometimes called "vaping".

The e-juice that fills the cartridges usually contains nicotine (which is extracted from tobacco), propylene glycol, flavorings, and other chemicals. Studies have found that even e-cigarettes claiming to be nicotine-free contain trace amounts of nicotine. Additionally, when the e-liquid heats up, more toxic chemicals are formed. Because the Food and Drug Administration (FDA) has not begun its review of any e-cigarette or its ingredients, nor has FDA issued any standards on the products, e-cigarette composition and effects vary. E-cigarettes have the potential to benefit adult smokers who are not pregnant if used as a complete substitute for regular cigarettes and other smoked tobacco products. However, e-cigarettes are not safe for youth, young adults, and pregnant women, as well as adults who do not currently use tobacco products. While e-cigarettes have the potential to benefit some people and harm others, scientists still have a lot to learn about whether.