The cartomizer is the portion of an electronic cigarette that contains the nicotine. This is the part of the e cigarette that you need to replace when the nicotine runs out.
In the early days when e cigarettes first came out, the cartomizer was actually made up of two parts. The atomizer and the cartridge, which is the part that held the nicotine. The atomizer was the part that had the heating element that would heat up and vaporize the nicotine solution into a water vapor.
Manufacturers and customers found that the atomizer and cartridge tend to leaked a lot. The atomizers were also poorly made and tend to break down a lot so manufacturers combined the two parts into one component and that is how the cartomizer got its name by atomizer and combining it with cartridge.
Today a cartomizer works very well because there is less nicotine leakage and the consumer get a better draw from the e cigarette when they smoke it.