The pro version has more features than the free one. Automatic navmesh calculation and layered grid graphs being the most obvious differences, but there are lots of other cool features as well as a few under the hood improvements. You can find a comparison page here. The list is large, but does not list every difference.
You can purchase the pro version in the Unity Asset Store. When you have purchased it, you will be able to download it in the Unity Asset Store, but you can also get access to beta versions by entering your invoice number on the download page. Beta versions contain new features and improvements which have not been released yet.
For simplicity, the license is the same one as the Unity Asset Store uses. Check the asset store for the exact specifications. It does allow for commercial usage.
The Pro Version costs $100. It includes free upgrades to all coming 3.x versions of the project.