Think of the OEM key as windows at a wholesale level and the retail well, err, at the retail level.
That is, the OEM keys are sold to desktop and laptop netbook, etc, manufacturers for installing it on their products and selling them.
That’s in short is the reason why OEM keys are more cheaper as compared to your windows license at a retail/ online etc.
Before you hop on and get an OEM key though, a few things you should know.
Any hardware change will leave your OEM key invalid that is no longer usable change such as your motherboard, and others.
And Microsoft doesn’t provide any kind of replacement for the OEM keys.