Originally introduced as an aid to help smokers quit, electronic cigarettes mimic the effects of real cigarettes. Containing nicotine, they produce a vapour that is "potentially" less harmful than cigarette smoke and are free of some of its damaging substances. Users refer to themselves as "vapers" - as opposed to smokers - and the vapour electronic cigarettes produce is all but odourless.
However, there is no hard evidence as to their safety and no-one knows what the long-term impact is on health.
The contents of e-cigarettes can vary. Some have been found to contain toxic chemicals which are also found in tobacco and have been linked to cancer. They come in a variety of flavours, mimicking well-known cigarette brands.
Others come in cherry or cheesecake flavour, which has led to warnings about teenagers taking up the habit.
Such is the public health concern that the Welsh government wants to restrict their use only to places where you can legally smoke tobacco - for example outside or in smoking shelters.