An OEM toner cartridge is a toner cartridge that has been made by the same company that made your printer. It's usually more expensive than aftermarket cartridges, but it's also usually of higher quality, and it will usually last longer.
Some people choose to buy OEM cartridges because they believe that they're less likely to experience compatibility problems with their printer. Others buy them because they believe that the quality of the prints will be better. Still, others buy them because they believe that the cartridges will last longer. Ultimately, it's up to the individual consumer to decide whether or not an OEM cartridge is worth the extra cost.