Different e-cigarettes are manufactured in different ways. They have some type of battery. Some are disposable, but you'd use them a number of times. Some you could refill many times with liquid, and you'd buy the liquid separately. Those would be the fundamental components.
I think that in the United States, the Food and Drug Administration is looking at what types of potential regulation can be done, and they've had a consultation process on that.
In the European Union, a new directive was adopted in April—and there may already have been testimony on this. The nicotine threshold will affect the type of regulation an e-cigarette has, whether it would be regulated as a medicine/drug product or have other types of regulations that would be more similar to tobacco products.