OEM stands for Original Equipment Manufacturer. If you buy a computer from Dell or such like the software you recieve with it (The Operating System or Office for example) are OEM verions of the software, It is also possible to buy these OEM verions of software (Which are cheaper than 'Full Retail' versions) from people like OcUK on the provisor that it is bought with components that constitute the core of a system, once installed with these components it can no longer be installed anywhere else. Also in theory if these components are changed a new licence must be purchased to use the OEM software.
If you have purchased a 'Retail' version of software, you own the licence which you can transfer as you need as long as you never exceed the number of licences you have purchased.