The chemical composition of the electronic cigarette aerosol varies across and within manufacturers. Limited data exists regarding their chemistry. However, researchers at Johns Hopkins University analyzed the vape clouds of popular brands such as Juul and Vuse, and found "nearly 2,000 chemicals, the vast majority of which are unidentified."

An electronic cigarette is a handheld battery-powered vaporizer that simulates smoking, but without tobacco combustion. E-cigarette components include a mouthpiece, a cartridge, a heating element/atomizer, a microprocessor, a battery, and some of them have an LED light on the end. An atomizer consists of a small heating element, or coil, that vaporizes e-liquid and a wicking material that draws liquid onto the coil. When the user inhales a flow sensor activates the heating element that atomizes the liquid solution; most devices are manually activated by a push-button. The e-liquid reaches a temperature of roughly 100–250 °C (212–482 °F) within a chamber to create an aerosolized vapor. The user inhales an aerosol, which is commonly but inaccurately called vapor, rather than cigarette smoke. Vaping is different from smoking, but there are some similarities, including the hand-to-mouth action of smoking and an aerosol that looks like cigarette smoke. The aerosol provides a flavor and feel similar to tobacco smoking. There is a learning curve to use e-cigarettes properly. E-cigarettes are cigarette-shaped, and there are many other variations. E-cigarettes that resemble pens or USB memory sticks are also sold that may be used unobtrusively.