Juul explained

This week I found a comprehensive review of Juul and vaping at Vox.  Herewith some excerpts:

E-cigarettes have quietly eclipsed cigarette smoking among adolescents. The possibility of another generation getting hooked on nicotine is a nightmare scenario health regulators are scrambling to avoid.

No device right now is as worrisome as the Juul — because of both its explosion in popularity and the unusually heavy dose of nicotine it delivers. In 2017, the e-cigarette market expanded by 40 percent, to $1.16 billion, with a lot of that growth driven by Juul...

While school administrators like Kenny are glad that cigarette smoke is disappearing from campuses, they’re concerned that students don’t understand the risks of using the Juul. What sets it apart from other e-cigarettes is that it hits the body with a tobacco cigarette-worthy dose of nicotine. We don’t know if Juul is more addictive than regular cigarettes, but it’s certainly possible that teens getting into

Juul now may be wading into a lifelong habit...

But the FDA, under Gottlieb, also delayed the compliance deadline for the regulation of e-cigarettes until 2022. This gave e-cigarette manufacturers who had products on the market before 2016, including Juul Labs, a free pass when it came to filing public health and marketing applications before selling in the US...

... they designed an e-cigarette that could easily be mistaken for a USB flash drive — and can fit in the palm of the hand. The Juul has two components: the e-cigarette, which holds the battery and temperature regulation system; and the “pod,” which contains e-liquid — made up of nicotine, glycerol and propylene glycol, benzoic acid, and flavorants — and is inserted into the end of the e-cigarette device. Pods come in a variety of colors and flavors, from cucumber to creme brûlée, mango, and tobacco. Juul’s “starter kit,” the e-cigarette, USB charger, and four flavor pods, sells for about $50. 

When you insert the pod into its cartridge and inhale through a mouthpiece on the end of the Juul, the device vaporizes the liquid. When the device runs out of power, you can connect it to your computer via a USB charger for a reboot...

But the biggest appeal for David is how discreet the device is. “You can essentially Juul wherever without drawing much attention.”  For Stewart, the student at Providence College, it’s also the flavors. “If they had bland flavors, then not as many people would [Juul].”

Each pod contains 59 milligrams of nicotine per milliliter of liquid. Juul claims one pod is equal to a pack of cigarettes in terms of nicotine, but tobacco experts told me the precise equivalency is difficult to determine because not all the nicotine released in cigarette smoke is inhaled, and some is trapped in the filter. Juul also contains three times the nicotine levels permitted in the European Union, which is why it can’t be sold there. 

There's more information at the Vox article.