OEM = it has to live and die on the pc its first installed on and is not transferable.
If you change your, mobo, or even cpu, you are required in accordance to the EULA to buy a new OEM copy of office (oh and your OS). How much does that suck!?!
RETAIL = Can be installed on any ONE pc at a time. Unless its for personal use in which case the regular user of it can also install it on a portable computer (laptop). You're not supposed to use both at the same time.
Theoretically you could install the same version on a few pc's and just blag MS when you phone them for activation. But i didn't tell you that....and besides it breaks the EULA if you do!
edit: standard and pro have the same EULA, so the same rules apply to both, basically its one pc per licence key.