Vapes -- also known as e-cigarettes -- are electronic devices that heat a solution (i.e, e-liquid) to produce an aerosol that is inhaled similar to cigarette smoke. Vapes are known by many names, including: e-cigs, e-hookahs, mods, vape pens, tank systems, and pod mods, or by popular brand names like JUUL.
When someone puffs on a vape, a sensor detects the puff and sends a signal to the battery. The battery turns on a mini-heater which heats the liquid nicotine. This creates an aerosol that is inhaled and then exhaled. The nicotine in vapes is absorbed through the lining of the mouth and throat, as well through the lungs.
Vapes use small cartridges (pods) or refillable tanks of e-liquid. The ingredients of e-liquids vary. Many e-liquids contain nicotine, an addictive stimulant. Other ingredients include propylene glycol, glycerin, water, and flavoring agents. E-liquids can also contain other substances, such as tetrahydrocannabinol (THC), the main psychoactive ingredient in marijuana.
Since vapes are currently sold without much regulation, e-liquid can be contaminated with unknown substances. There have been reports of illness and even deaths among people who vape, although the specific cause of these incidents are still under medical investigation.
Randomized trials have shown that vapes can function as a nicotine replacement therapy to improve smoking cessation rates1. However, the population effects of vapes are still being hotly debated2. Vapes are not currently approved by the U.S. FDA or included in the U.S. Public Health Services Clinical Practice Guideline for treating tobacco use and dependence, in part because the safety of the products themselves has not been established.
 Hajek P, et al. A Randomized Trial of E-Cigarettes versus Nicotine-Replacement Therapy. N Engl J Med;
January 30, 2019 doi: 10.1056/NEJMoa1808779.
 Public Health Consequences of e-Cigarettes. Washington, DC: National Academies of Sciences Engineering and Medicine; January 23, 2018.