This is a useful page on the different ways that Windows 10 can be activated, ranging from the new “digital entitlement” when you’ve used the free upgrade to Windows 10, to the traditional product keys required when Windows 10 is purchased through a Volume Licensing agreement.

There’s also help on what to do when you need to activate after reinstalling Windows 10 or when you’ve made a hardware configuration change.

Read all about it here: http://bit.ly/1JMzIEG.