Generally OEM referrs to the packaging and lisence and has no effect on the capabilities of the software.
An OEM copy of an operating system will generally stipulate that it is only valid if purchased with a PC.
Also a retail copy usually comes with a certain number of free tech support calls whereas an OEM copy comes with none.
OEM copies also tend to have not much in the way of packaging. Usually a short manual and the CD shrinkwrapped together.
Finally if I am not mistaken, a retail copy is valid for two PCs. One desktop and one Laptop. An OEM copy is only valid for the PC it was purchased with. Microsoft may have changed the lisence agreement on the retail copies to be only for one PC since I last checked though, so doublecheck before you install it on both.
Hope this helped.