Electronic cigarettes, at least the majority of them, consist of 3 components: battery, heating element, and liquid.
The batteries are generally rechargeable and last at least a few hours, if not days.
The heating element is generally called an atomizer, "cartomizer" or "clearomizer". Basically a small container holding 1-5ml of liquid has a heating coil of some sort in it along with a wicking material that, when heated up, the coil heats up the wick, atomizing the liquid, turning it into vapor.
The main ingredients in USA made e-liquid are propylene glycol and/or vegetable glycerin, natural/artificial flavoring extracts, and nicotine.
Studies so far have shown that "second hand" vape poses no threat to the "second hand" inhaler.
The FDA is starting to consider regulating them as a "tobacco product" is regulated but its the furthest thing from it.