Keys are usually stored in the TPM chip and/or tied to Microsoft accounts if you use one. If you don't have an account, there's actually a limit to how often a key can activate new hardware. If there's no TPM, there's simply a limit within a certain timeframe that it can be used to activate on the same hardware.

Retail keys can also be linked to the hardware or Microsoft account

Super late reply to your question, but when you add Windows and punch in the key it links to the motherboard of the PC (laptop or desktop, doesn't matter). Once you sign in to your Microsoft account, the key and mobo serial gets stored there. If you bought a new PC and with it a new motherboard you can find your key in your account or use the one on the back of your old laptop and delink it from your old device so that you can reuse it on your new one. It moves with your account.

Of you do a standard format, you shouldn't have to worry, but if you ever upgrade your mobo, just make sure you get a screenshot of your serial in the windows settings and make sure you are logged in to your windows account (maybe check the account settings on a browser and see if you can find your devices, not sure of this is a necessary step.. but just to make sure it's linked I guess).. then once you format, you can punch in the old serial and log in, if it pops up a warning, it may ask you to migrate the serial and delete the old device or you may have a step or two on the Microsoft website to do this, I'm a bit hazy on the details as I did this a good 4 years ago, bit it wasn't much of a headache.