Also known as Electronic Cigarette. Other names are Personal Nicotine Vaporizer, Personal Vaporizer and Vaporizer. The product usually consists of 3 major parts - A rechargeable battery, an atomizer, and a cartridge with nicotine (or no-nicotine) liquid.
E-Cigs are products which allow the user to vaporize liquid, usually water based (with other ingredients), laced with nicotine. The result allows one to inhale "smoke" that feels similar to the act of smoking tobacco cigarettes, and get one's nicotine absorption in the process.
The battery is triggered when sucked on. The combination of the battery being on, and the forward motion of sucking, heats up the "atomizer" which has a wick and coil. The unique heater that the atomizer is, can take liquid on the wick and transform it into tiny particles which become vapor. That is then inhaled like traditional cigarette smoke.
The multiple angles mimicking traditional smoking such as inhaling, handling a unit similar in shape and length to a tobacco cigarette, exhaling and seeing vapor, and absorption of nicotine are usually pleasing to tobacco cigarette smokers, in a way that leads them to choose smoking e-cigarettes over tobacco ones.