Electronic cigarettes or electronic nicotine delivery systems (ENDS) are devices that do not burn or use tobacco leaves but instead vaporize nicotine which the user then inhales. These products are most commonly referred to as e-cigarettes and using e-cigarettes is commonly referred to as ‘vaping’. Although some e-cigarettes look like conventional tobacco products (e.g. cigarettes, cigars, cigarillos, pipes, hookahs, or shishas), they may be produced in the form of everyday items such as pens, USB memory sticks and larger cylindrical or rectangular devices.

The other main constituents of the fluid besides nicotine are propylene glycol and vegetable glycerine. In addition, e-cigarette liquids and emissions usually contain other chemicals. E-cigarettes come in a variety of flavours.

Update: IARC endorses the World Health Organisation (WHO) statement based on the current scientific evidence: 

WHO: E-cigarettes: how risky are they? 

The link to the WHO statement posted on 29/01/2020 by the IARC provides recent information related to this Question and Answer of the European Code against Cancer posted on 14/10/2014. The European Code against Cancer Working Group on Tobacco developed this Question and Answer but had no involvement in the development of the WHO statement.