Hi
OEM Original Equipment Manufactuer. you get the Disk thats it, no support, no warranty no help should it go wrong.
Full retail version: you get a disk, instructions, warranty leaflet, support numbers, but no ..Bill Gates wont come round and install for you..
OEM's are usually included with a branded PC, where you go to the PC manufacturer should there be a problem.
Full Retail Versions are for those who have a PC and want to change it to a new OS and will need support and advice from an MS representative.
as for which you need, if you know PC's go for the OEM, if your a numpty with no clue as to what an OS is go for the retail.
Regards