compressed from: http://h30507.www3.hp.com/t5/Coffee-Coaching-HP-and-Microsoft/What-Windows-Server-2008-R2-end-of-sales-means-for-your-SMB/ba-p/172060#.VdjXQbS-aTJ
Windows Server 2012 R2 (Foundation, Essentials, Standard, and Datacenter). To help you better understand your options as Windows Server 2008 R2 reaches its end of sales, let’s compare the current WS2012 R2 editions.
Microsoft Windows Server 2012 R2 Foundation is for single processor servers and comes with 15 user accounts. CALs are not required for access. The Foundation edition can be implemented in environments where features such as file sharing, printer sharing, security, and remote access are required.
Microsoft Windows Server 2012 R2 Essentials (the latest version of Microsoft Windows Small Business Server Essentials) is an ideal option for small businesses with up to 25 users. Essentials provides a powerful, flexible solution that helps protect business data, while allowing users to access the information they need from virtually anywhere, using almost any device. This edition is for either one or two processor servers and does not require CALs for access.
Microsoft Windows Server 2012 R2 Standard is great for physical computing or lightly virtualized environments. Standard provides virtualization rights for up to 2 virtual instances along with the full feature set of the Datacenter edition. The Standard edition is able to support an unlimited amount of users, as long as required user licenses have been purchased.
Microsoft Windows Server 2012 R2 Datacenter is ideal for highly-virtualized private clouds. Datacenter and Standard editions are differentiated only by virtualization rights—two virtual instances for Standard edition and unlimited virtual instances for Datacenter edition. Both editions have a consistent processor-based licensing model that covers up to two physical processors on a single server.