If you didn't move up from Windows 7 or 8 during the year it was a free upgrade, you can still get Windows 10, but you have to pay.
Windows 10 is an Editors' Choice desktop operating system.
Pro adds business-y things like network domain joining, Hyper-V virtualization, group policy management, and Bit Locker encryption.
That last one may be of interest to security-conscious personal users, too.
Unsurprisingly, if you upgrade from Home levels of Windows 7 or 8, you get Windows 10 Home, and if you update from the professional versions of 7 or 8, you get Pro.
There are, of course, other editions of Windows 10 for special use cases: The latest is the already-mentioned Windows 10 S.