E-cigarettes comes in two parts. In one end there is liquid nicotine, in the other a rechargable battery and an atomiser. When the user sucks, the liquid nicotine is vaporised and absorbed through the mouth. What looks like smoke is largely water vapour.
Because there is no tobacco in e-cigarettes, there is no tar and it is the tar in ordinary cigarettes that kills.
The e-cigarette market is growing fast. Some medical experts see huge potential benefits.
Nicotine itself is not a particularly hazardous drug. It is something on a par with the effects you get from caffeine.