1. OEM License: The least expensive and best way to buy Original Microsoft Software is to get it preinstalled when you are buying a PC. The license you get is an OEM License.
2. FPP License: A Fully Packaged Product (FPP) refers to the retail pack that you can buy online or from a retail store or get it delivered to your door-step. FPP is most suited when you need Original Microsoft Software for 1-4 PCs.
3. Volume License: For 5 or more PCs, it is best to go for Volume License. Based on your requirements, a Volume License Partner will guide you on the right type of Volume License.

Typically, you can also purchase Software Assurance (SA) along with your Volume License to get the latest updates and versions.
To know more about Microsoft licensing in general, click here.