There are actually two types of OEM versions:
1.) OEM copies pre-installed by the manufacturer are usually customized by said manufacturer and include various third-party applications (such as trial security software, a DVD player, burning software, a word processing application, etc.), as well as the manufacturer's logo and contact information. They also either come with or require that you burn a set of system recovery discs, which include not only the OS but the added software and customizations as well. This software is locked to the specific model of computer and cannot be installed on another machine.
2.) OEM copies purchased through online retailers and, occasionally, in retail stores. They are designed for system builders, and are just a copy of the OS, identical to the full retail version. However, they come with several limitations:
* Once installed on one computer it can never be installed on another machine, nor are you permitted to replace the motherboard.
* You receive no technical support from Microsoft.
* You can only perform a clean installation of the OS, not an upgrade.
That is why these copies are usually $100+ cheaper than their retail counterparts...the licensing terms are far more restrictive, making the copies less versatile.
Hope this helps,