A cartomizer is a small metal cylinder with a preinstalled atomizer. The cylinder is filled with a polyfill wicking material (similar to cotton). E-liquid gets absorbed into the wicking material and vaporized by the atomizer. Cartomizers are not meant to have a long lifespan, and will typically need to be replaced after a few uses. Cartomizers were very popular when e-cigs were first introduced to the market, but have been largely overshadowed by numerous improvements to vaping technology over the past few years.
When purchasing cartomizers, you'll see different options available for resistance, size, and holes. Cartomizers come in a few different sizes (Standard, L, XL, XXL), which defines the length of the tube, and therefore its overall capacity. You will see options for either Standard (2.7Ω - 3.1Ω) or Low (2.0Ω - 2.4Ω) Resistance. These days, both of these resistance options are actually considered to be pretty high. You will also have an option to choose the number of punches, or holes, on your cartomizer. 0 hole cartomizers will have extremely limited airflow, and are intended for standalone use. 1 and 2 hole cartomizers will have better airflow, and are meant to be installed in a carto-tank.