Original equipment manufacturer, I think - basically its the same as a retail product minus the trimmings, probably in a brown box and in the case of a cpu will just be the CPU, no cooler, no fancy packaging.
beaten: For the CPU its a good option if you planned to use an after market cooler anyway.
A full retail copy of windows can be installed on any PC you like (but only one at a time), an oem install needs to stay on 1 PC only so if you blow your motherboard it might mean buying another windows, you can still make some system changes though