The difference is in the level of support provided by microsoft the oem version is for supposed to be for people who are going to be building a pc and selling it to someone else with the builder providing support to the buyer instead of microsoft. That said a lol of people who build there own systems uses the oem version because its normally 20 to 30 dollars cheaper than the retail disk . The retail disk comes with tech support from microsoft thats really the only difference both the retail and the oem disk are priced the same at the moment so there really isn't any reason not to go with the retail disk.
Neither is the retail version from what I can tell. In the product detail for both GFC-02050 and GFC-02733, it mentions that they are oem version. The listing at top there is difference, one (full), the other (oem). When you read product decription on both you get the same statement about oem version for builder and no Microsoft support. They seem to be the same thing, maybe just new packaging? So the question is what is the difference between GFC-02050 & GFC-02733.