OEM stands for Original Equipment Manufacturer. You've probably seen this term used a lot when shopping for a compatible ink cartridge. For example, if you have a Hewlett-Packard brand of inkjet printer and buy an ink cartridge made by Hewlett-Packard, you are purchasing what is considered an OEM cartridge. On the other hand, a compatible inkjet cartridge is made by another company, but is guaranteed to work in your Hewlett-Packard printer. Of course the original manufacturer wants you to purchase only their brand of consumable products, any business would in order to increase their sales and profits. But to save money, you can purchase a compatible ink cartridge instead and get the same results. You should also know that compatible ink cartridge companies such as those listed on this website usually sell both OEM and compatible inkjet cartridges. They are so sure the compatible brand will work in your printer that they guarantee it or offer a warranty. However, most OEMs do not allow these companies to warranty their name brand products, therefore, you will need to deal directly with the OEM if you need to make a return or an inkjet cartridge you receive does not work for some reason.