An electronic cigarette is a battery powered device that vaporizes a nicotine solution that the user can inhale. It does not contain tobacco or create smoke. The nicotine solution contains a solvent (usually propylene glycol), nicotine and flavoring.

The battery has an LED on the end that lights up when the user draws on the mouthpiece.

The atomizer is in the middle of the electronic cigarette, this part vaporizes the nicotine solution and creates a mist that the user inhales.

The mouthpiece or cartridge is the business end, this is the part that contains the nicotine solution and the part that the user sucks on to create the nicotine vapor for inhalation. 

Some electronic cigarettes combine the mouthpiece and atomizer into a single disposable unit. This design cuts down on atomizer problems but also requires the user to buy the more expensive non-refillable combination units. Vendors claim that a single cartridge is the equivalent of 7-20 cigarettes depending on the style and brand.  

Nicotine solutions are often called "e-liquid" or "smoke juice" and come in various flavors such as tobacco, coffee, chocolate, and vanilla.There are several different manufactures of e-liquid and dozens of flavors on the market. Each flavor is usually available at various nicotine levels ranging from 0 to 8 percent.

HEALTH ISSUES
Nobody knows the health effects of using electronic cigarettes. There have been no extensive peer reviewed studies on the devices and they are not approved by any US government agency or the World Health Organization as either a smoking cessation aid or as a safer alternative to traditional cigarettes. There have been some toxicological studies that suggest that electronic cigarettes contain fewer harmful chemicals than traditional cigarettes and vendors of often claim that they are safer. The vapor produced by e-cigs does not contain many of the major toxins found in traditional cigarette smoke.

LEGAL
Electronic smoking devices are legal in most countries and smoking bans generally do not apply to them since they do not contain tobacco and do not create smoke. The vapor exhaled by users resembles smoke but quickly dissipates and produces little or no lingering odor.

ADVANTAGES
Compared to traditional smokes e-cigarettes produce no ashes or litter, create no smoke, and don't leave behind an odor. They can also be used in places where smoking is usually not permitted. However using them in places where smoking is not permitted can cause confusion and/or agitation for people around the e-smoker who are not familiar with the devices.