1) A rechargeable Lithium Ion Battery
2) Nicotine and flavoring, suspended in a liquid state (primarily water and Propylene Glycol or Glycerin: All FDA approved food ingredients). These liquids are known as e-Juice.
3) An small absorbant strip of fabricwhich holds the e-Juice.
4) An eCigarette Atomizer which heats the liquid and atomizes it, this is where the vapor which looks like smoke is produced.
5) The actual e-Juice.
6) A mouth piece.
E-Cigarettes are available in many different packages, from numerous manufactures, however there are two primary styles. One style has only two parts, the other style is incorrectly referred to as a three part system. The three part system actually contains six parts, five of which must be replaced from time to time. It's pretty simple: the Two part eCigarette is better than the Three part eCigarette (actually a 6 part system). It's just that simple.
The two-part e-Cigarette consists of a rechargeable Ion Lithium battery. The battery comes with a lifetime warranty from the manufacturer. The battery is usually white and looks like the tobacco portion of a real cigarette.
The second part is usually a light brown cork color, to emulate the filter of a standard cigarette. This part is a disposable cartridge which contains:
1) The mouth piece
2) The liquid nicotine and flavoring (e-Juice)
3) The absorbent felt which is saturated with the e-Juice
4) The atomizer.
E-Cigarettes are available in many different packages, however there are two primary styles. One style has only two parts, the other style has three parts. Two is better than three. It's simpler.
The two-part e-Cigarette consists of a rechargeable Ion Lithium battery. The battery comes with a lifetime warranty from the manufacturer. The battery is usually white and looks like the tobacco portion of a real cigarette.
The second part is usually a cork brown color, to emulate the filter of a standard cigarette. This part is a disposable cartridge which contains the liquid nicotine and flavoring plus an atomizer.
There are numerous drawbacks to using an old fashioned 3-piece e-Cigarette. First, you must manually add two separate liquids, the nicotine e-juice and the flavoring. You don't have to be a chemist to mix these two liquids, but you do need to adhere to the manufactures guide lines when combining the two products. 
Next, you also really need to very careful whenever you come in direct contact with the concentrated liquid nicotine. In this concentrated form, it is poison. If you have children or pets, make sure you store these supplies where they do not have access to them.
Whenever you handle e-juice, be sure to thoroughly wash your hands before and afterwards. Even better yet, wear disposable rubber gloves.
A 3-part e-Cigarette has a battery similar to the 2-part e-Cigarette. The second part of this 3-part e-Cigarette is the atomizer, this atomizer is designed to be reused. The atomizer is an accessory and is sold separately. The atomizer in this 3-part e-Cigarette requires constant cleaning and eventually will become so clogged up that it will need to be replaced.
The third part is a reusable cartridge, this cartridge must be constantly refilled manually with both nicotine and flavoring. After the cartridge has been used several times, it will also become clogged up and will need to be replaced. You can purchase additional cartridges from the manufacturer.
There are numerous problems with this 3-part design. You will know when the atomizer and/or the nicotine cartridge needs cleaning, it gets harder and harder to obtain an easy puff.
Anyone who has smoked wants a smooth and easy draw whenever we take a drag on our e-Cigarette. The two-part system is much safer and definitely easier to use. Since the atomizer plus the liquid nicotine and flavoring are all pre-packaged in a disposable cartridge for you. You get a brand new atomizer every time you screw on a disposable cartridge.