All electronic cigarettes are made up of the same basic parts. Although there are variations in design, all function in the same manner. Here is a brief explanation of the different parts that make up an e-cigarette and some great places to get replacement parts.
The Battery
All electronic cigarettes must have a power source. There are 3 basic designs for the battery.
The first and most common is a rechargeable lithium ion battery. This is the largest, longest part of the e-cigarette and sometimes has an LED at the end that is activated when the user inhales to mimic the glowing cherry at the end of a traditional tobacco cigarette.
The second type of battery is a considered a mod (modification). It has a housing where you can insert rechargeable batteries. In this case the batteries generally need to be purchased separately. If you have or plan to buy this type of mod, make sure the batteries you get are “protected” or they can blow up.
The third type of battery is a passthrough. In this case, it is not really a battery, but rather hooks directly into a USB port to provide power to your e-cig. Of course it never runs out of power but you do need to have a computer handy to make it work.
Batteries can also come in automatic or manual. Automatic batteries are activated when you inhale. Manual batteries require you push a button while inhaling. Some people like manual batteries because as a rule they are able to produce more vapor. Also as a general rule, the bigger the battery, the more power it holds and the less you have to charge it.
Battery Chargers
Many brands offer charges that can be plugged into the wall, a USB port, or a car cigarette lighter. There are also portable battery chargers available that allow you to charge a cigarette pack sized box which then houses your e-cigarette battery and charges it on the go. Some companies also offer solar powered battery packs.
Atomizers and Cartridges or Cartomizers
The second part to an e-cigarette other than the battery is a heating element and someplace to store e-liquid. There are a few different designs, but the main ones are models that use separate atomizers and cartridges and the ones that use a combination of the two; called the cartomizer. In both cases, the cartridge or the cartomizer could also be called the mouthpiece.
The atomizer/cartridge setup was one of the first designs. The atomizer is simply a little heating element while the cartridge holds the e-liquid. In the past, the cartridges have used a polyester batting to absorb the e-liquid and hold it until it is vaporized, but in the past couple of years the new tank design has come out. This is a cartridge but holds the e-liquid in a tank and uses some sort of feeding mechanism to deliver the e-liquid to the atomizer. Tanks are generally considered better because they hold more e-liquid and are easy to refill.
Cartomizers are just cartridges with a disposable heating element inside the cartridge. This is the simplest design and one that many brands have moved toward. They are super simple to use because the cartomizer just screws onto the battery and you are ready to go. The problem is they are not meant to be refilled so once one is used up, it needs to be replaced with a new cartomizer. This is more expensive than refilling cartridges, but is still much cheaper than smoking cigarettes.
Green Smoke Parts
Green Smoke offers different sized batteries, in different colors, with different colored LED tips.
They also have USB Passthroughs:

Here are their battery charger options:
Green Smoke uses cartomizers. They come in a variety of flavors and nicotine strengths.
V2 E-Cigarette Parts
A guide to V2 Cig batteries:
V2 USB Passthroughs:

V2 Cig charger options: