Main difference is Retail license is for use on a single system but is transferable as long as you are only using it on one system at a time and support is offered through Microsoft if you have any problems ---- OEM is licensed for one system only and once the MOBO dies the license expires (it can not be moved to a new system (or is not supposed to be)) and Microsoft does not provide support for any issues (other than security) if you need support it is provided by the OEM that installed it not Microsoft. SO the extra money that you pay for the retail version is for support from MS and for being able to move oit from system to system as you change systems rather than having to repurchase each time you change systems.
JDFan is right. But I would just like to add that microsoft doesn't seem to care if you move a OEM license from one machine to another. I've done it a bunch of times. Worse case is you have to call to activate and even then it's usually all automated.