This is a useful site if you want to learn more about product activation: http://bit.ly/1CoqHlm.

It includes good information on all the basics as well as answering less well-known questions such as: which activation method should be used for virtual computers, what key to use on PCs preinstalled with Windows when you want to downgrade to an earlier version, and why the amount of MAK activations doesn’t match the number of purchased licences.