The two biggest differences is
The first major difference is
1) By the letter of the license agreement, OEM are tied to the motherboard of the computer that it is activated on. Where as retail, you are allowed to move it to another system
For example, if you build a computer today, and use the OEM version, if you build a new computer next year for example, you will have to buy another license.
But for retail, you can move the license from the old to new, so long as you format the old one as you cannot use the same license on two machines
2) Oem version the support is done by you/system builder vs ms. this one does not matter so much as 99% of the stuff most people would google anyway