Microsoft requires you to enter a long serial number when you install Windows. Which is then checked against a database to make sure it's legit. If it isn't, you are a pirate. If it is, you are good. Apple doesn't deal with silly long serial numbers because they know only Mac users will have the OS. And it comes with all their computers. And even now with Hackintosh's they haven't resorted to the draconian method of making sure the OS is legitimately purchased and not being installed on a hundred computers.
I don't know if they still do it, but there was also a time when you couldn't change too much hardware without the OS screaming at you for being a dirty pirate and trying to transfer your OS to another computer.
Apple uses the honor system. You can install one OS X disc on a hundred computers if you were so inclined, but the small number of people who are that evil is outweighed by the amount of legit users who will buy a Family license for their 5 Macs.
Microsoft on the other hand will make sure you buy a hundred copies of Windows (Or their new family pack, how many PC's can a family pack of 7 be installed on) to install on your hundred PC's. Even though every PC comes with a copy of Windows, they still want a brand new license when you upgrade every time. Making you type in a hell of a lot of seemingly random strings of letters and numbers.
I used to use Windows in the 90's, and the part I hated every single time was entering those goddamned serial codes.
While Microsoft would rather punish everyone for the small amount of pirates, Apple knows the amount of good people outweighs the amount of bad ones.
Just one of those conveniences that reminds me why I love this company.