As someone who did much research on the topic (even though I’m not a smoker)
1) They work by evaporating the solvents propylene glycol and/or glycerin. Being that these have a high boiling point, they quickly condense into an ultra-fine aerosol. These solvents also are widely used in the food industry as flavoring carriers. They are also widely used in medicinal inhalers (which work differently).
2) They’re orders of magnitude less dangerous than is smoking, based on toxicological studies and some tests for acute cardiac and pulmonary clinical effects. There are no major known side effects for those switching. Though, some may experience dry-throat.
Two cases of lipoid pneumonia were reported – due to someone putting oil into the liquid.
The most important toxicological review on the topic is titled “Peering Through the Mist..”
3) You can get EC w/o nicotine, however there’s a significant difference in cessation levels among EC with nic and w/o.
4) There’s almost no level of continuous use among youth who never smoked. Experimenting among never-smokers is rare (~0.5%).