Atomizer is the part of the E-Cigarette in between the battery and the cartridge. The Atomizer is the heart of the E-Cigarette, because it contains a micro computer that converts the liquid in the cartridge into vapor which is then inhaled by the user. Atomizers can ware out over time so it is always a good idea to keep a spare. Also if you decide to drop E-Liquid directly on the atomizer so that you get a more realistic feeling then you will definitely want to keep a spare. (For more information on this please read the Tips section of the FAQ).