Original Equipment Manufaturer.
OEM copies of Windows are intended to be installed by computer manufacturers on the computers they build. They come with very basic packaging and you will not have any telephone support from Microsoft... they will tell you that you have to contact your system manufacturer for any support issues. OEM copies of Windows are also intended to be installed only on one machine (same as retail), but the difference is if you sell the computer, that copy of Windows must go with it... no wiping and installing it on another computer. This is why OEM copies are much cheaper than retail copies.
With retail copies, you are allowed to uninstall the OS from the current PC and install it on a completely different PC. You could do this with OEM copies as well... but it is a violation of MS's license agreement.