CO2 cartridge is a small metal cylinder holding the highly pressurized CO2 gas to inflate a bicycle tire in a matter of seconds with little effort. CO2 cartridges come in 12g, 16g, 24g, 25g. There are two types - threaded and non-threaded CO2 cartridges. A threaded one is the best pick for the first-timers.
To make the CO2 cartridge work you also need a CO2 inflator/nozzle. Make sure the CO2 cartridge and the inflator kit are compatible with each other. For instance, a threaded CO2 cartridge goes with the threaded CO2 inflator/nozzle and vice versa.