MSDN is for Software developers. It allows them to use the majority of Microsoft products for development purposes.
OEM is a licence that is attached to a computer and isn't transferable. So for example, you bought a HP desktop and it came with an OEM Vista Licence. You can't then use this licence on another computer.
RTM = Release to Manufacturer. This refers to when a piece of software is "finished". As in, it is no longer in testing or a beta so it has been RTM so that physical discs can be produced. It is therefore not a licence as such.
Enterprise = In the Microsoft world this is a particular type of licencing that large (enterprise) businesses may use. It has all sorts of requirements and is far too complex to explain here.
Hope those short descriptions are all you need.