Difference between Free and Pro listed here.
* Unlimited PCs in the same home means that you can activate all your personal home PCs. This applies only to a single home/family and, commercial use is not allowed. The Server Edition is exempt. For practical purposes, you may see a limit expressed such as ‘5 PCs’, but you can disregard that and know it means unlimited within your home.
If you have a credit voucher or discount code, it will be reflected on the final checkout page (not here).