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 kinds are reusable. The vapor mainly comprises 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 less harmful than smoking, but still harmful. E-cigarette vapor contains fewer toxins than cigarette smoke. It contains traces of harmful substances not found in cigarette smoke.

Nicotine is highly addictive. Users become physically and psychologically dependent. Scientists do not how harmful e-cigarettes are over the long-term because it is hard to separate the effects of vaping from the effects of smoking when so many people both vape and smoke. E-cigarettes have not been used widely enough or for long enough to be sure.

For people trying to quit smoking, e-cigarette use alongside prescribed nicotine replacement therapy leads to a higher quit rate. For people trying to quit smoking without medical help, e-cigarettes have not been found to raise quit rates.