Full is as it says. The whole shooting match able to be installed on a PC with no previous windows OS. Can be moved to any PC you own as long as it's only on one machine at a time.
Upgrade version requires a proscribed earlier version of windows to install - probably 98, ME, 2000 or NT
OEM is a version that in theory can only be purchased with a piece of hardware such as a new pc or a new hard drive. Often only has a recovery CD, especially when purchased with a new PC. Again, in theory, it should only be used with that specific hardware as that is what the OEM licences specifies.
Full version keeps your long term options more open. But it is a bit on the dear side!