An electronic cigarette is an electronic device that simulates tobacco smoking. It consists of an atomizer, a power source such as a battery, and a container such as a cartridge or tank. Instead of smoke, the user inhales vapor. As such, using an e-cigarette is often called "vaping". 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 look like traditional cigarettes, and most versions are reusable.

The vapor is primarily composed of propylene glycol and/or glycerin, usually with nicotine and flavoring. Its exact composition varies, and depends on several things including user behavior.

Vaping is likely significantly less harmful than smoking tobacco, although it nevertheless carries health risks. The life expectancy of tobacco smokers is at least 10 years shorter than for nonsmokers. E-cigarette vapor contains fewer toxins, in lower amounts, than cigarette smoke. Nicotine is addictive, but by itself is not greatly harmful. For people trying to quit smoking with medical help, e-cigarettes have a higher quit rate than normal nicotine replacement therapy. For others, e-cigarettes do not raise quit rates.