Users inhale the way they would with a regular cigarette and this activates the atomiser to heat the liquid and convert it to an odourless vapour.
The vapour is inhaled like a normal cigarette, delivering nicotine to the lungs, and a small electric light comes on at the end of the device.
The liquid in the cartridge is usually propylene glycol, an additive that has been approved for use in food and is also used in smoke machines.
They contain differing amounts of nicotine or no nicotine and usually have flavouring, such as menthol or fruit. One e-cigarette cartridge is the equivalent of around 40 cigarettes. The Electronic Cigarette Industry Trade Association claims that while the start-up costs can be high, over a year, e-cigarettes tend to be around one-quarter of the price of regular cigarettes.
Users say they enjoy the devices as they offer many of the same features as tobacco products - inhaling and exhaling, the ability to hold the device and the light at the end of the stick simulating a flame.