With a battery, an atomizer and a cartridge filled with a liquid nicotine solution, electronic cigarette is known as a device that releases vaporized nicotine. It works in the similar way by using the battery to power e cigarette and heat up liquid with the help of atomizer. Some parts are disposable, other parts such as batteries are rechargeable.

Battery - it is an initial part to power e-cig and heating elements. The battery life largely depends on its type, size, use frequency and environment.  
Atomizer / Cartomizer - an atomizer consists of heating element for liquid heating up and vapor production, so vaporizing nicotine can be inhaled. It should be changed every 3 to 6 months.
Cartridge - It is installed to the end of tube, which is replaceable and refillable. The nicotine strength depends on the amount of liquid it holds. 
E liquid - the main components of e liquid is a mix of PG, G and PEG400. Some contains alcohol and flavors. Most e liquids contain different levels of nicotine.

Electronic cigarette looks, tastes, feels, and acts like a traditional cigarette. When you exhale vaporized nicotine, you seem to enjoy smoking regular cigarette. But it is free of tobacco, smoke and other harmful chemicals. 

In 2003 e cigarette was first invented by Hon Lik, a pharmacist from China. And e-cig was first emerged in Chinese market in 2004. From 2005 to 2006 e cigarettes have been exported to many foreign countries. By the year of 2014 there have been three generations. 

The First Generation of E-Cigarette
The appearance of first generation is similar as a regular cigarette, which is also called “cig-a-like”. The device contains a battery and a single tube filled with e juice. Whether battery or juice is used up, the e cigarette will be discarded. But there is a reusable design of cartomizer that separates battery and cartridge. The battery can be charged when it dies. The cartridge can be changed when e juice is running out.  

The Second Generation of E-Cigarette
Second generation e cigarette is more popular among experienced e smokers. The device is larger than traditional cigarette. With higher capacity, the batteries can be charged by a USB charger. A tank or clearomizer is used to fill and refill e juice. 

The Third Generation of E-Cigarette
The third generation has mechanical mods and variable voltage devices, which is also called “mods”. It can contain larger and more batteries, which allows the larger tank to fill with more e juice. The e-cig battery can be removed and rebuildable atomizers give users chances to DIY coils for increasing vapor production.